Hello,
Without troubleshoot it is difficult to tell the reason.
However, one of the things that you can try is to configure automatic modem reboot on data disconnection event, without restarting the whole device. To configure, navigate to Services -> Auto Reboot -> Ping/Wget reboot. There you can select to reboot modem or restart mobile connection if no ping replies are received from defined hosts. You can read more about it here.
Best regards