First, I would like you to update to the latest 7.4.3 firmware with Keep settings option disabled. Firmware can be downloaded from here.
Also, you are using automatically obtained APN setting. Could you disable Auto APN can in router's WebUI Network -> Interfaces -> General page, by editing mobile interface settings, disabling Auto APN slider button and, selecting APN option value as --Custom--, then entering APN according to your provider.
One more thing you could try is to configure periodic pings to some remote host for connection health checks and automatically restart connection or modem in case of disconnect. To do this, login to router's WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Create a new or edit existing default instance, enable it, set Action if no echo is received to Restart mobile connection, adjust other settings, if needed, otherwise, leave default values.