Could you please confirm that you've entered the SIM card PIN code in router settings? You can find the field to enter a PIN code by navigating to Network>Mobile. Additionally, try using different service mode settings, for example "3G + 4G" or "4G (LTE) only".
If your issue still persists even after entering the SIM card PIN and a custom APN then please generate the troubleshoot file and send it to me via private message. Also, please include your internet provider in the message.
What's a troubleshoot file and how to generate it?
A Troubleshoot file contains a device's event logs, configuration files and other info useful for diagnostics. It can be downloaded from your device's WebUI, Troubleshoot page:
System → Administration → Troubleshoot