General advice if you are writing firmware for your device:
- Setting up automatic carrier/operator selection will allow the modem transition carriers freely. Different modems set this up differently, but AT+COPS=0 is a common way to configure this. Just make sure that the APN is configured correctly.
- For using our SIMs, it is important that the device be setup to enable roaming, setting a home network preference will not work. (AT+CREG returning 0,5 is normal).
- Allow for long scanning periods when reconnecting. Scanning for towers can take up to 3 minutes (typically resolve in 30-60 seconds). Stopping a scan early can prevent a tower from being sensed and prevent the device from selecting it.
- If you are deploying in areas without particular network coverage (i.e. NB-IoT or 2G), consult your modem manual to see if you can change the radio access technology preference to avoid spending time scanning for these networks (which can take a long period of time and will always fail if your country does not have support for those networks).
- Periodically check the FPLMN list and/or clear this list at reboot, this will prevent your device prematurely blocking a carrier in the event of a network event (tower failure, congestion, etc returning a denied access). You can refer to this guide for clearing the FPLMN.