General advice if you are writing firmware for your device:

  1. 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.
  2. 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)
  3. 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.
  4. 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).
  5. 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.
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