Sooner or later somebody tries to connect to a device on a mobile SIM from the office and finds they cannot. The device has internet access, the signal is fine, and yet nothing gets through. That is not a fault. It is how mobile networks are built, and fixed IP SIMs and private APNs are the two standard ways around it. Here is what each one does, in plain English, and when you genuinely need to pay for them.
The 30-second version
- Most mobile SIMs cannot be reached from the internet. They sit behind carrier grade NAT, with no public address of their own.
- A fixed IP SIM gives the device a permanent address that stays the same, so it can be reached or allow-listed.
- A private APN keeps the SIM traffic off the public internet and delivers it into your own network.
- Many devices need neither, because they connect outward to a cloud service.
- Both cost more, so buy them for a reason rather than by default.
On this page
Why you cannot reach your device in the first place
When a SIM connects, the network gives it an address. On almost every consumer and standard business mobile plan, that address is private and shared, sitting behind what the industry calls carrier grade NAT. Traffic can go out, and replies come back, but nothing can start a connection inward.
It is the same reason you cannot connect directly to a phone by its address. For most uses that is fine and safer. It becomes a problem when the device is a camera recorder, a controller or a monitoring unit that somebody needs to reach on purpose.
What a fixed IP SIM gives you
A fixed IP SIM, sometimes called a static IP SIM, gives that SIM an address that does not change and that can be reached from outside, or at least be recognised by systems that care about where traffic comes from.
- Inbound access. Remote support, camera viewing or engineering access to equipment that has no cloud service.
- Allow-listing. Some systems only accept connections from known addresses. A fixed address makes that possible.
- Consistency. The address stays the same through reboots and SIM swaps, so documentation stays accurate.
Reachable means exposed
If a device is reachable from the internet, it is reachable by everyone, not just you. Anything on a public fixed IP needs a firewall, current firmware and sensible passwords. This is exactly the kind of equipment that gets found by automated scanning.
What a private APN does differently
An APN is the doorway a SIM uses to reach the network. Standard SIMs use a public APN that delivers traffic to the internet. A private APN sends that traffic somewhere else: into a dedicated path that reaches your own network, often over a VPN or a private circuit.
In practice that means the devices behave as if they are on your network rather than on the internet. You can address them directly, apply your own firewall policy, and keep them off the public internet entirely.
| Standard SIM | Fixed IP SIM | Private APN | |
|---|---|---|---|
| Reachable from outside | No | Yes | From your network only |
| Traffic path | Public internet | Public internet | Private path to you |
| Typical use | Phones, tablets, most IoT | Cameras, remote equipment | Estates of devices, sensitive data |
| Setup effort | None | Low | Planned, with your IT |
| Relative cost | Lowest | Higher | Highest |
Do you actually need one?
Most connected equipment does not, and that is worth saying plainly because both options cost more. Work through these questions in order:
- Does the device connect outward to a cloud platform? If yes, you almost certainly need neither. Most modern cameras, trackers and sensors work this way.
- Do you need to start a connection to the device? If yes, a fixed IP is the simplest answer.
- Does a third party need to allow-list you? A fixed IP is usually the requirement.
- Is the data sensitive, or the estate large? A private APN is the tidier and safer answer at scale.
- Is there an alternative? A modern remote access service, or a VPN from the device, can achieve the same result without a fixed address.
Where we see each one used
- Fixed IP: camera recorders at remote sites, building management controllers, older telemetry, machines a supplier maintains remotely.
- Private APN: estates of payment terminals, utility telemetry, healthcare equipment, and anywhere the customer does not want devices touching the public internet at all.
- Neither: vehicle trackers, vending telemetry, environmental sensors, digital signage, most modern IoT.
Our fixed IP SIM
We supply a business fixed IP SIM that combines the two things this article covers most often: a fixed address the device can be reached on, and multi-network coverage so it stays connected where a single network struggles. It comes with unlimited UK and EU data, so a camera recorder, a controller or a router at a remote site is not sitting on a small allowance.
It suits equipment that has to be reachable and cannot afford to drop: CCTV at unmanned sites, remote plant and machinery, routers at temporary premises, and anything a supplier maintains remotely. Tell us what the device is and where it lives and we will confirm whether it is the right fit, or whether a standard SIM would do the same job for less. If the device in question is a camera recorder, our guide to what we check when site CCTV keeps dropping out covers the usual causes.
Not sure which you need?
Tell us what the equipment is and who needs to reach it. We will tell you honestly whether a fixed IP or private APN is needed, or whether a standard SIM will do the job.