Quick answer: A public IP SIM gives a remote cellular device an internet-reachable address. That can be useful, but it also means exposed services may be discoverable by scanners, search engines and attackers unless access is locked down properly.
Why public IP SIMs became popular
Public IP SIMs solved a real problem. Installers needed to reach CCTV recorders, routers, BMS controllers, solar inverters, telemetry equipment and remote HMIs over 4G. A static public IP made that easy. You could configure port forwarding, bookmark the address and connect from the office.
The problem is that easy access for you can also mean easy discovery for somebody else. Internet scanning is constant. Exposed login pages, old firmware, weak passwords and open management ports are found quickly.
The common mistake
The biggest mistake is treating a 4G router like a private cable. It is not. If the SIM has a public IP address and inbound traffic is allowed, the router is part of the wider internet. That means the same security expectations apply as they would to a public web server.
Better access models
For many sites, the better model is VPN-first access. The remote device uses a private or dynamic address, then establishes an outbound tunnel to a secure platform. Engineers connect to the platform first, authenticate properly, then reach only the devices they are allowed to access.
When a public IP SIM still makes sense
There are cases where a public IP SIM is still useful, especially for specialist systems with strict firewall rules, source IP whitelisting, strong passwords, updated firmware and no exposed admin interfaces. But it should be a deliberate engineering decision, not the default because it is easy.
Minimum checks for any public IP deployment
- Disable WAN access to router admin unless absolutely required.
- Use VPN or source IP whitelisting wherever possible.
- Change default passwords before installation.
- Keep router and connected device firmware updated.
- Use non-standard access methods only as a small extra layer, not as real security.
- Regularly scan your own IPs for exposed services.
The practical takeaway
Public IP SIMs are not automatically bad. Unmanaged public exposure is the problem. If a device does not need to be visible to the whole internet, it probably should not be.