IoT roaming SIM cards, M2M SIM cards, and multi-network SIM cards are sold with a promise of automatic network switching and guaranteed uptime. The reality is more complicated. This guide covers everything – from how PLMN selection actually works, to FPLMN lists, steered vs unsteered SIMs, permanent roaming, dual IMSI, eUICC, and SGP.32. By the end you will understand these products better than most people selling them.
- What is an IoT roaming SIM? What about M2M SIM and multi-network SIM?
- The sales pitch vs the reality
- UK mobile networks and what your SIM actually has access to
- Permanent roaming – the hidden risk in long-term deployments
- How PLMN selection works – the standard behind network choice
- The FPLMN list – why devices get stuck and how to clear it
- Steered vs unsteered SIMs – the question that matters most
- Why ping reboot doesn’t fix PLMN problems
- Real-world failure scenarios
- SIM management and router management platforms
- Four connectivity architectures compared
- Multi-IMSI and dual IMSI SIMs
- eUICC, eSIM and SGP.32 – the future
- LPWAN alternatives: LoRaWAN, NB-IoT, LTE-M
- Practical solutions by deployment type
- Buyer’s guide: questions to ask before you buy
- Frequently asked questions
What is an IoT Roaming SIM Card? M2M SIM? Multi-Network SIM?
These terms are used interchangeably across the industry. The distinctions matter:
| Term | What it means | What it doesn’t guarantee |
|---|---|---|
| Roaming SIM card | Any SIM that can register on networks other than its home network. | Multi-network access, automatic switching, or resilience features. |
| Multi-network SIM | A SIM with roaming agreements covering multiple operators – in the UK, some or all of EE, Vodafone, O2 and Three. | How many networks are actually included, or whether the SIM is steered. |
| M2M SIM card | Machine-to-Machine SIM. Industrial-grade, managed via a connectivity platform, available in MFF2 solderable form, rated for extended temperatures. Can be single or multi-network. | Multi-network access or automatic failover. M2M describes the industrial format, not the network configuration. |
| IoT SIM card | Broadly synonymous with M2M SIM in modern usage. Sometimes implies NB-IoT or LTE-M support. | Again, does not automatically mean multi-network or automatic switching. |
| Dual IMSI / Multi-IMSI SIM | A SIM holding multiple subscriber identities. An applet switches between them based on rules. Each IMSI has its own network relationships. | Instant switching – IMSI changes require a modem restart (30-90 seconds). |
| eUICC / eSIM | A SIM that can download and switch operator profiles over the air without physical SIM replacement. | Real-time automatic failover. Profile switching is managed, not autonomous. |
Bottom line for this guide: “IoT roaming SIM,” “M2M SIM,” and “multi-network SIM” are used throughout to describe SIM cards with access to multiple networks used in unattended devices. Where specific distinctions matter, they are called out explicitly.
The Sales Pitch vs the Reality
The pitch is familiar: “If EE goes down, the SIM switches to Vodafone or O2. You get four networks in one SIM.” A CCTV installer fits twenty-four cameras on a multi-network roaming SIM. Three months later, a regional EE outage takes down all twenty-four simultaneously. The supplier says it should have switched. It didn’t. Nobody can explain why.
This scenario repeats across the UK every week – in CCTV deployments, EV chargers, payment terminals, vending machines, traffic sensors, and agricultural equipment. The problem is not the SIM. It is a gap between what these products can do, and what they are sold as doing.
| What you were sold | What actually happens |
|---|---|
| “Automatically switches to the best network” | The modem follows a priority list on the SIM at registration time. It does not continuously compare performance and switch dynamically. |
| “If EE goes down it switches to Vodafone” | Only if EE’s radio signal disappears entirely. If EE has signal but broken data, the modem stays on EE – it cannot detect data failure. |
| “Four networks in one SIM” | Sometimes. Many M2M SIM cards only include two or three of the four UK networks. Always confirm which operators are actually included. |
| “Reboot the router and it reconnects on a working network” | Usually reconnects on the same network. The modem stores the last registered network (RPLMN) and tries it first on every restart. |
| “Multi-IMSI SIM switches instantly when a network fails” | IMSI switching requires a modem stack restart – 30 to 90 seconds of downtime per switch. Not instant, and most implementations do not trigger on data failure. |
The most important sentence in this guide: A roaming SIM or M2M SIM gives the modem permission to use multiple networks. It does not give the modem the ability to detect broken data and switch automatically. Those are two completely different things – and the industry has sold them as one for over a decade.
UK Mobile Networks and What Your M2M SIM Actually Has Access To
The UK has four Mobile Network Operators who own their own radio access infrastructure:
Note: Vodafone and Three completed a merger in 2025. Both brands continue as separate networks during integration – for SIM roaming agreement purposes, treat them as separate MNOs for now.
| SIM configuration | EE | Vodafone | O2 | Three | Practical implication |
|---|---|---|---|---|---|
| Full four-network | ✓ | ✓ | ✓ | ✓ | Maximum fallback options. Best for UK-wide deployments across varied site locations. |
| Three-network (EE + VF + O2) | ✓ | ✓ | ✓ | ✗ | Common and good. Three has urban 5G strength but rural gaps – this combination covers most UK sites well. |
| Three-network (VF + O2 + Three) | ✗ | ✓ | ✓ | ✓ | Weaker option for rural deployments – EE has the broadest UK rural coverage footprint. |
| Two-network only | varies | varies | varies | varies | Sold as “multi-network.” Significant gaps possible. Always confirm which two. |
What each network actually brings to an M2M deployment:
- EE leads on 4G geographic coverage, particularly rural UK. The default preferred network for most steered M2M SIM providers. Best choice as primary for remote or rural deployments.
- Vodafone strong on business-grade infrastructure, second to EE in UK national coverage. Excellent private APN support for enterprise M2M.
- O2 widest 5G Standalone footprint and lowest Ofcom complaint rate. Strong urban performance. Good for urban and suburban deployments needing 5G NR.
- Three competitive on urban 5G throughput and pricing. The VodafoneThree merger will strengthen rural coverage over time. Currently weakest of the four for remote rural deployments.
Always confirm in writing: “Which UK MNOs does this SIM have active roaming agreements with? Please confirm EE, Vodafone, O2 and Three specifically.” Do not accept “all major UK networks” – this phrase has no standard definition in the M2M SIM market.
When Your SIM is Homed Overseas: KPN, Orange France, Tele2
Some M2M SIM cards sold in the UK are issued by operators based outside the UK – KPN (Netherlands), Orange France, Tele2 (various European markets). When a KPN-homed SIM operates in the UK, every session is international roaming. This has consequences:
- Data travels a longer path – from the UK visited network back to the KPN core in the Netherlands, then out to the internet
- The PLMN priority list reflects KPN’s commercial roaming agreements, not the best network at your site
- Roaming agreements between KPN and individual UK MNOs are commercial contracts that can change – as the KPN/O2 UK 2G and 3G withdrawal in late 2025 demonstrated, when O2 UK removed inbound 2G/3G access for KPN SIMs with limited advance notice
- Permanent roaming restrictions apply – see the next section
Permanent Roaming – The Hidden Risk in Every Long-Term Deployment
Roaming was designed for travelling consumers who come home after a few weeks. IoT and M2M devices do not return home. A smart meter installed in Manchester with a KPN SIM roams on UK networks for its entire operational lifetime – potentially ten to fifteen years. This is permanent roaming, and it creates serious risks for IoT deployments.
The 30 to 90 Day Limit
Under most bilateral roaming agreements and a growing body of national regulation, a SIM can remain outside its home network for between 30 and 90 days before being classified as permanently roaming. At that point, the visited network may terminate the connection. Under EU regulation, devices must spend the majority of any four-month period on their home network to avoid being classified as permanently roaming – a condition that IoT devices can almost never meet.
| Region | Restriction | IoT impact |
|---|---|---|
| Brazil | ANATEL-enforced 90-day hard limit. | Devices must localise to a Brazilian operator profile after 90 days or face disconnection. |
| China | Permanent inbound roaming banned. Local licence required. | Foreign SIMs cannot operate long-term. Local profile mandatory. |
| Turkey | Permanent roaming not permitted. | Local operator agreement required. |
| EU | Devices must spend majority of any four-month period on home network. | Affects IoT using non-EU-homed SIMs in EU deployments. |
| Australia / USA | No regulatory ban but many MNOs enforce commercial permanent roaming restrictions. | Risk of connection termination when MNO policies change. |
| UK (post-Brexit) | No hard ban. Individual MNO commercial policies apply. | Risk from MNO policy changes (e.g. KPN/O2 UK 2025). Monitor agreement health. |
For any deployment expected to run for more than two years, ask your M2M SIM supplier: “What is the home network of this SIM? What happens if a UK MNO changes its inbound roaming agreement with the home operator? Do you offer a migration path to a locally-homed or eUICC-based profile?” If they cannot answer clearly, treat long-term connectivity as at risk.
How PLMN Selection Works – the Standard Behind Every Network Choice
Every mobile network has a unique code called a PLMN – Public Land Mobile Network identifier. UK MNO codes:
| Network | MCC | MNC | PLMN |
|---|---|---|---|
| EE UK | 234 | 30 | 234-30 |
| Vodafone UK | 234 | 15 | 234-15 |
| O2 UK | 234 | 10 | 234-10 |
| Three UK | 234 | 20 | 234-20 |
When any cellular modem powers on – in a router, camera, GPS tracker, or M2M device – it runs a PLMN selection procedure governed by 3GPP Technical Specification 23.122. The procedure is sequential and rule-based, not performance-based:
- RPLMN first: The Registered PLMN – the last network the modem was on. Stored in modem memory. Tried first on every startup before evaluating any alternatives.
- HPLMN / EHPLMN: The home operator network as encoded on the SIM.
- User PLMN selector (EF_PLMNwAcT): Networks listed in the SIM’s user-controlled PLMN file, in priority order set by the SIM issuer.
- Operator PLMN selector (EF_OPLMNwAcT): Networks in the operator-controlled file, also in priority order.
- Other available PLMNs: Any remaining visible networks by signal strength.
Critically absent from every step: data quality, ping response, internet reachability, latency, or any measure of whether a network is delivering working connectivity. The modem picks a network based on radio signal and a priority list. Once registered, it considers itself connected – regardless of whether data flows.
Signal bars measure radio signal strength at the antenna (layer 1). Working internet connectivity requires a functioning APN session, operator packet core, and backhaul routing (layers 4-5+). A network can show four bars and have completely broken data routing. The modem cannot detect this – it operates at the radio layer with no visibility into the data path above it.
The RPLMN Trap: Why Rebooting Returns You to the Same Broken Network
The RPLMN is the most misunderstood aspect of M2M router behaviour. After every successful registration, the modem writes the current PLMN to RPLMN memory. On the next restart – power cycle, ping reboot, firmware update, anything – the modem tries the RPLMN first.
If EE was the last network and EE still has radio signal at the antenna, the modem registers on EE again – without evaluating whether EE’s data was the problem. This is why during an EE outage, an entire fleet that was registered on EE will return to EE after every reboot, fail immediately, and cycle indefinitely.
The FPLMN List – Why Devices Get Permanently Stuck and How to Clear It
The FPLMN – Forbidden PLMN list – lives on the SIM card itself in a file called EF_FPLMN, not in the modem. When a modem tries to register on a network and that network actively rejects the attempt, the modem writes that network’s PLMN code to the FPLMN list. Once listed, the modem will not attempt that network again during automatic selection mode.
Because the FPLMN lives on the SIM card:
- It survives router reboots and power cycles
- It survives replacing the router hardware – the SIM carries the list to the new device
- It is not cleared by ping reboot, interface restarts, or most watchdog features
- A SIM that fills its entire FPLMN list shows “No Service” and will not recover without intervention
Common causes of FPLMN entries being written include temporary network rejections during congestion, lapsed roaming agreements, network switch-offs sending rejection cause codes (as with the KPN/O2 UK 2G/3G withdrawal), and steering conflicts causing rejection loops.
Checking and Clearing the FPLMN via AT Commands
Do not clear the FPLMN in a loop. If a network is genuinely rejecting the SIM, clearing the list restarts the rejection cycle and causes excessive signalling to the network. Clear once, observe the result, investigate the root rejection cause before clearing again.
Steered vs Unsteered SIMs – The Question That Matters Most
| Feature | Steered SIM | Unsteered SIM |
|---|---|---|
| Network priority | Priority list on SIM – preferred operator always tried first regardless of signal quality | No commercial preference – modem selects based on available signal |
| Who sets the priority | SIM issuer, reflecting wholesale roaming commercial agreements | No preference set – or equal priority across all available networks |
| Behaviour under poor signal | Stays on preferred network until signal drops below minimum threshold – even when better options exist | Evaluates alternatives earlier – better in marginal signal environments |
| Steering of Roaming immunity | Usually not immune – home network can push OTA SoR messages reinforcing the preference | Should be immune – but confirm. True immunity requires a specific SIM applet. |
| Best suited to | Sites where a specific network is confirmed best by survey and the operator wants guaranteed attachment to it | Fixed IoT, M2M, CCTV deployments across varied UK locations |
Steering of Roaming (SoR) means the home operator can send an OTA message to the SIM instructing it to switch to a specific visited network – typically a commercial partner. A genuinely steering-immune SIM has an applet that intercepts and ignores SoR messages. Most commodity roaming SIM cards do not have this protection, even ones marketed as unsteered.
Why Ping Reboot Doesn’t Fix PLMN Problems
Ping reboot monitors IP connectivity and restarts the router when pings fail. The problem of a modem stuck on the wrong PLMN lives two layers below that:
When ping reboot fires and restarts the router, the modem reinitialises and runs PLMN selection – finds the RPLMN still present at layer 1, registers on the same network, re-establishes layers 3 and 4 on the same broken network, and layer 5 fails again. The cycle repeats indefinitely.
On Teltonika routers, Low Signal Reconnect (Network > Mobile) operates closer to the modem level – it triggers modem-level reconnection when signal drops below a configured threshold, making PLMN reselection more likely. Use both: ping reboot for IP-layer failures, Low Signal Reconnect for signal-degradation scenarios.
Real-World Failure Scenarios
EE regional outage – signal present, data broken
EE has a backhaul fault. All devices on EE have radio signal but no working data. Pings fail, ping reboot cycles, modems return to EE on RPLMN. EE still has signal. Loop continues until EE restores service.
FPLMN list fills after rejection loop
Temporary network faults cause repeated registration rejections. Networks are written to FPLMN one by one. Eventually the SIM has no networks to try. Device shows “No Service.” The FPLMN is on the SIM – rebooting the router does nothing.
Steered SIM on poor-performing preferred network
EE-preferenced steered SIM at a rural site where Vodafone actually has better coverage. Modem keeps returning to EE. Connection is marginal indefinitely. Supplier says “the SIM always picks the best network.” It doesn’t.
Overseas SIM after UK roaming agreement change
A UK MNO changes or terminates its inbound roaming agreement. Overseas-homed SIMs lose access to that network. If it was the primary PLMN, devices enter rejection loops, accumulate FPLMN entries, and need manual intervention.
30% of fleet goes dark – no visibility
Without a SIM or router management platform, you discover the outage when customers call. You cannot see which devices are on which network, cannot identify the EE-affected subset, and have no remote remediation path.
Stale APN session or crashed interface
The data bearer session has become stale or the network interface has crashed. Restarting the interface re-establishes the APN session on the same network. This is what ping reboot was designed for and it does well.
SIM Management and Router Management Platforms
Two different management layers are needed for complete fleet visibility. Most deployments have neither – and discover outages only when customers call.
What a Good SIM Management Platform (CMP) Provides
- Real-time online/offline per SIM – the foundation of proactive fleet management
- Current network operator per SIM – during an EE outage, filter to “all SIMs currently on EE” and see exactly which devices are affected
- Data usage monitoring – zero data for 24 hours signals an offline device; ten times normal data signals a configuration problem
- OTA FPLMN management (where supported) – push FPLMN clear commands to the SIM via SMS when a device has no data connection. Not all SIM providers offer this – ask specifically
What even the best SIM platform cannot do: see the router’s CPU or interface state; execute AT commands on the modem; configure watchdog settings; distinguish “SIM offline” from “router has no power.” The SIM platform sees the SIM – the router hardware is a black box to it.
Router Management Platforms: RMS and Milesight Development Platform
A router management platform covers the hardware layer. Teltonika RMS and the Milesight Development Platform both maintain an outbound connection from the router to a cloud server, providing remote access through dynamic IP and CGNAT.
With a router management platform, when 30% of the fleet goes dark you can: see immediately which devices are offline; check the current operator on each device; SSH into affected routers and check FPLMN status; execute AT commands to clear FPLMN and trigger modem reinitialisation; and monitor reconnection in real time – all without leaving a desk.
The critical gap today: even with both platforms deployed, most recovery logic still requires human decision-making. The platforms provide visibility and tools. A person interprets the data and acts. Autonomous remediation – detecting “30% of fleet on EE is offline, clear FPLMN on affected devices” without human instruction – is buildable with existing APIs but is not yet a standard product offering.
Dynamic IP vs Fixed IP SIMs
Most M2M SIM cards use dynamic IP addresses – the device gets a new IP each session. You cannot reach a dynamic IP device from outside when it stops reporting. Solutions: a fixed IP SIM card (permanent public IP, inbound connections work at any time); or a router management platform that maintains an outbound tunnel through which remote access is delivered regardless of IP address. See our Fixed IP SIM cards guide for UK options.
Four Connectivity Architectures Compared
Traditional roaming SIM
Single IMSI. Roams on visited networks via bilateral agreements. PLMN priority list on SIM. Subject to RPLMN, FPLMN, steering, and permanent roaming risk.
- Home network controls priority via SIM files
- Permanent roaming risk on long deployments
- Exposure to agreement changes
- No flexibility post-deployment
Multi-IMSI / Dual IMSI SIM
Multiple subscriber identities on one SIM. Applet switches between IMSIs based on configured rules. Each IMSI has its own network relationships.
- Better per-region coverage and local presence
- 30-90 second downtime per IMSI switch
- Universal APN required for seamless switching
- Provider-locked IMSI library
Network-agnostic core (specialist MVNO)
Provider builds direct core network integrations with all RANs globally. The SIM is never technically roaming – all RANs are treated as home networks. No bilateral roaming agreements needed.
- Eliminates permanent roaming risk entirely
- Single IMSI, single APN globally
- No PLMN priority steering
- Dependent on provider’s RAN relationships
eUICC with SGP.32
eUICC holds multiple operator profiles. Remote profile management via SGP.32. New operator profiles pushed OTA without physical SIM replacement.
- Remote operator switching at fleet scale
- Local profiles eliminate permanent roaming
- Standards-based (GSMA SGP.32)
- Ecosystem still maturing
Multi-IMSI and Dual IMSI SIMs Explained
A multi-IMSI SIM holds multiple IMSI values on a single physical card. A SIM applet acts as a rules engine, presenting different identities to the modem based on location, signal, timer, or platform instruction. Each IMSI has its own operator relationships – so the device can appear as a local subscriber on different networks rather than always roaming.
What multi-IMSI genuinely improves: better network agreements per region; can appear as a domestic subscriber on the preferred network rather than a roaming visitor; reduces permanent roaming exposure; provides an alternative identity when the current IMSI’s network rejects the SIM.
What multi-IMSI does not fix:
- Switch speed: IMSI switching requires a full modem stack restart – 30 to 90 seconds of downtime per switch. Not instant.
- Switch triggers: Quality varies enormously. Many implementations only switch on complete signal loss – the same condition that triggers standard PLMN reselection. Better implementations trigger on data connectivity failure, which is meaningfully more useful.
- APN continuity: If APN configuration changes with the IMSI, the device may lose data during the switch. A universal APN that works across all IMSIs is essential.
- FPLMN per IMSI: Each IMSI can accumulate its own FPLMN entries independently.
- Provider lock-in: The IMSI library is fixed by the provider. You cannot add operator relationships post-deployment. For true flexibility, eUICC is required.
eUICC, eSIM and SGP.32 – the Future of IoT SIM Connectivity
eUICC technology changes the model at its root. Rather than a SIM with fixed identities attempting to roam, an eUICC can hold multiple complete operator profiles and switch between them over the air – without physical SIM replacement.
| Standard | Designed for | Relevance to IoT M2M |
|---|---|---|
| SGP.02 | Industrial M2M | Widely deployed. Operator-push model. Complex infrastructure requirements. |
| SGP.22 | Consumer devices (smartphones) | Requires a user interface. Not suited to headless IoT devices. |
| SGP.32 | Headless IoT devices | The correct standard for M2M. Designed for devices with no UI, constrained power and bandwidth. |
SGP.32 (GSMA, 2023) introduces the eIM (eSIM IoT Manager) – a management layer that pushes new operator profiles to devices asynchronously without requiring a user interface. In practical terms: when EE has a persistent regional outage, a fleet operator can push a Vodafone or O2 profile to all affected devices remotely – new network credentials, new PLMN configuration, new APN settings – without any site visit.
eUICC caveats: the eUICC platform is hardcoded at manufacture – you cannot change eSIM platform providers post-deployment. Full SGP.32 ecosystem adoption is still developing; not all industrial routers have SGP.32-capable SIM slots, and not all MNOs have SM-DP+ infrastructure for SGP.32 profile delivery.
The hybrid approach that works now: load a multi-IMSI bootstrap profile on an eUICC-capable SIM for immediate multi-network coverage, with SGP.32 management for longer-term profile changes as the deployment matures.
For more on eUICC, see euicc.co.uk. For SGP.32 in depth, see sgp32.co.uk.
LPWAN Alternatives: LoRaWAN, NB-IoT and LTE-M
Not every M2M application requires a full cellular SIM. LPWAN technologies offer an alternative connectivity model that sidesteps many of the roaming SIM issues described in this guide:
| Technology | Licensed | Data rate | Roaming issues | Best for |
|---|---|---|---|---|
| LoRaWAN | No (ISM bands) | Very low | None – operates outside MNO ecosystem entirely | Sensors, meters, infrequent small data payloads |
| NB-IoT | Yes (cellular) | Low | Same PLMN/FPLMN/permanent roaming issues as 4G | Low-power metering, sensors on licensed spectrum |
| LTE-M (Cat-M1) | Yes (cellular) | Low-medium | Same cellular roaming issues. LTE-M roaming agreements less complete than 4G. | Mobile IoT, asset tracking with voice capability |
| 4G LTE | Yes | High | All PLMN/FPLMN/roaming issues apply | CCTV, routers, high-data M2M |
LoRaWAN is the one genuine escape from the cellular roaming problem for suitable applications. If your device sends small data packets infrequently – sensor readings, meter values, status pings at intervals of minutes or longer – LoRaWAN may eliminate the multi-network SIM problem entirely by operating on unlicensed spectrum outside the MNO ecosystem. NB-IoT and LTE-M, despite being designed for IoT, are cellular technologies subject to exactly the same PLMN selection and roaming restrictions as standard 4G.
Practical Solutions by Deployment Type
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1UK fixed-site IoT (CCTV, EV chargers, sensors): unsteered four-network SIM + correct router config
Unsteered, steering-immune, all four UK networks confirmed, universal APN. Low Signal Reconnect configured alongside ping reboot. Operator whitelist set after site survey. For critical sites, dual SIM on separate MNOs with SIM switch rules.
-
2Fleet of 50+ devices: deploy SIM management platform + router management platform
At this scale, manual management fails before the first outage. Real-time per-device online/offline status, current network per SIM, and remote AT command access are operational necessities, not nice-to-haves.
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3Critical infrastructure (SCADA, substations, BESS): dual SIM on separate MNOs
Dual SIM routers with EE on SIM1 and Vodafone on SIM2. SIM switch rules on ping failure. No shared RPLMN, FPLMN, or APN state. When SIM1’s network fails, SIM2 connects on a completely independent modem context. The most reliable hardware-level resilience available today without eUICC.
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4Global or long-lifecycle deployments (5+ years): multi-IMSI on eUICC
Multi-IMSI gives immediate multi-network coverage from day one. eUICC with SGP.32 allows remote operator profile updates as the deployment matures – localising to avoid permanent roaming restrictions, or migrating operators when agreements change, all without physical access.
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5Infrequent small payloads (sensors, meters): evaluate LoRaWAN
If your device sends data at intervals of minutes or longer with small payload sizes, LoRaWAN may eliminate the roaming SIM problem entirely. No MNO relationships, no PLMN rules, no permanent roaming risk. Battery life measured in years.
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6Remote access requirement: fixed IP SIM
For devices that need inbound connections – camera feeds, router web UI, VPN endpoints, direct SCADA connectivity – a fixed IP SIM gives the device a permanent public or private IP address. Dynamic IP SIMs cannot receive inbound connections.
Buyer’s Guide: Questions to Ask Before Buying Any IoT or M2M SIM Card
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Which UK networks does this SIM have access to? Confirm EE, Vodafone, O2 and Three individually.
“All major UK networks” is not a specification. Get the specific MNOs confirmed in writing.
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Is the SIM steered or unsteered? Is it immune to Steering of Roaming messages?
Steered SIMs prefer a commercial partner network regardless of signal quality. True SoR immunity requires a specific applet.
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What is the home network? Is it UK-homed or permanently roaming from an overseas operator?
KPN, Orange France, Tele2 and other non-UK-homed SIMs are technically always roaming on UK networks. Ask what happens if a UK MNO changes its inbound roaming agreement.
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What triggers a network switch and how long does it take?
Signal loss only, or data connectivity failure? Seconds or minutes? A SIM that switches on data failure is genuinely more valuable than one that only switches on complete signal loss.
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Does it use a universal APN across all networks and IMSIs?
If the APN changes during a network or IMSI switch, the device may need reconfiguration – and many routers will not do this automatically.
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What does the SIM management platform show per device? Does it show current network operator?
Online/offline alone is insufficient. Current MNO per device lets you identify which SIMs are affected by a specific network outage.
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Can you OTA-clear the FPLMN list when a device has no data connection?
This is the recovery capability that matters for devices stuck on a full FPLMN list. Without it, a stuck device requires physical access or router management platform AT command access.
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Is the SIM eUICC-based? Does it support SGP.32?
For deployments with a lifetime beyond 2-3 years, or any global deployment, eUICC with SGP.32 is worth specifying. It is the only current path to remote operator migration without physical SIM replacement.
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Do you offer fixed IP options and what is the difference in cost?
Fixed IP SIMs are essential for any device that needs to be reachable from outside – camera access, remote router management, VPN endpoints, SCADA connectivity.
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Has this SIM been tested with the specific router model I am deploying?
APN compatibility, IMSI switch behaviour, and watchdog interaction all vary between modem chipsets and firmware versions. Test on actual hardware before deploying at scale.
Frequently Asked Questions
A roaming SIM is any SIM that can register on networks other than its home network. A multi-network SIM specifically implies access to multiple operators – in the UK, some or all of EE, Vodafone, O2 and Three. All multi-network SIM cards are technically roaming SIMs, but not all roaming SIMs are multi-network. The terms are used interchangeably in the M2M industry. Always confirm which specific networks are included rather than relying on either term at face value.
The most common cause: the modem stayed on EE because EE still had radio signal at the antenna, even though its data path was broken. The modem registers on the RPLMN (the last known network) first on every restart, and it has no visibility into whether data is actually flowing above the radio layer. If the SIM is also steered to prefer EE, it will keep returning to EE even when alternatives are available. Ping reboot does not fix this because it operates at the IP layer – the modem returns to the same RPLMN after every restart.
PLMN stands for Public Land Mobile Network. Every mobile network has a unique PLMN code (EE UK is 234-30, Vodafone UK is 234-15, O2 UK is 234-10, Three UK is 234-20). When a modem selects a network, it selects a PLMN. The PLMN selection process (3GPP TS 23.122) follows a strict priority order based on the SIM’s configuration and the modem’s memory of previous registrations – not data quality or network performance. This is the root of why IoT roaming SIM cards often stay on failing networks.
The FPLMN (Forbidden PLMN) list is stored on the SIM card in a file called EF_FPLMN. When a network actively rejects a SIM’s registration attempt, the modem writes that network’s PLMN code to this list. The modem will not retry listed networks during automatic selection. The list survives reboots because it lives on the SIM, not the modem. To check it via SSH on a Teltonika router: AT+CRSM=176,28539,0,0,12 – 24 Fs means the list is empty. To clear it: send AT+CFUN=0, then AT+CRSM=214,28539,0,0,12,"FFFFFFFFFFFFFFFFFFFFFFFF" (exactly 24 Fs), then AT+CFUN=1.
A steered SIM has a PLMN priority list encoded at manufacture that causes the modem to prefer a specific operator – typically because the SIM issuer has a commercial wholesale agreement with that network. An unsteered SIM has no such preference and allows the modem to select based on available signal. For most fixed M2M and IoT deployments across varied UK locations, unsteered is significantly better. A genuinely unsteered SIM should also be steering-immune – protected against Steering of Roaming (SoR) messages from the home operator.
Permanent roaming occurs when an M2M device uses a SIM registered to a foreign operator for its entire deployment lifetime, without ever returning to the SIM’s home network. Roaming agreements were designed for travelling consumers, not permanently deployed IoT devices. Many countries restrict or ban permanent roaming – including Brazil (90-day limit), China (complete ban), and Turkey. The EU has regulations requiring devices to spend the majority of a four-month period on their home network. For UK deployments using overseas-homed SIMs, individual UK MNO policy changes can also terminate roaming access with limited notice.
SGP.32 is a GSMA standard (2023) for remote SIM provisioning on IoT devices. It allows eUICC SIM cards to download and switch operator profiles over the air without physical SIM replacement or a user interface. This enables fleet operators to remotely switch an entire device estate from one operator to another – resolving permanent roaming issues, responding to outages, or migrating operators – all without physical access. See sgp32.co.uk for full detail on SGP.32 and its industry applications.
A multi-IMSI SIM (sometimes dual IMSI SIM) holds multiple International Mobile Subscriber Identities on a single physical card. A SIM applet switches between identities based on rules – location, signal, timer, or platform instruction – allowing the device to appear as a local subscriber on different networks rather than always roaming. The key trade-off is that switching IMSIs requires a modem stack restart (30-90 seconds downtime per switch), and the available IMSI library is determined by the SIM provider, not changeable post-deployment. For more on IoT SIM types, see our IoT SIM cards guide.
Key Takeaways
- A roaming SIM or M2M SIM gives the modem permission to use multiple networks. It does not give the modem the ability to detect broken data and switch automatically. These are two different things sold as one.
- Not all UK multi-network SIMs include all four MNOs. Confirm EE, Vodafone, O2 and Three individually in writing.
- PLMN selection follows 3GPP TS 23.122 based on radio signal and priority lists – not data quality. The modem returns to the RPLMN on every restart. Signal present plus broken data equals the modem staying on the failing network indefinitely.
- The FPLMN list lives on the SIM card, survives reboots and router replacements, and silently accumulates forbidden networks. Clearing it requires AT commands or OTA SIM management.
- Steered SIMs prefer a commercial partner network regardless of signal quality at the site. Unsteered, steering-immune SIMs are the correct choice for fixed M2M and IoT deployments.
- Overseas-homed SIMs (KPN, Orange France, Tele2) are permanently roaming on UK networks. Permanent roaming restrictions apply in an increasing number of countries, and UK MNO policy changes can terminate access without warning.
- Ping reboot monitors IP layer only. PLMN problems live two layers below that. Low Signal Reconnect on Teltonika routers is more appropriate for signal-degradation scenarios. Use both.
- Both a SIM management platform and a router management platform are needed for complete fleet visibility. Each covers different blind spots. Without both, you find out about outages when customers call.
- Dual SIM on separate MNOs with SIM switch rules is the most reliable hardware-level resilience for critical deployments today.
- Multi-IMSI / dual IMSI SIMs are better than basic roaming SIMs but have specific trade-offs. eUICC with SGP.32 is the correct long-term architecture for multi-year or global deployments.
- LoRaWAN eliminates the cellular roaming problem entirely for applications with small, infrequent data payloads.
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