Which firmware version is you device using?
You could try to configure an Auto reboot of the modem module in case of the disconnect instead of restarting the device. To do so, in the WebUI navigate to Services -> Auto reboot -> Ping/Wget reboot and add a new instance with Action set to Modem reboot.
It would be more helpful if, after the next similar occurrence, you would generate a troubleshoot file. At the moment, it is difficult to estimate, the logs in the file would reveal more details.
To generate the file, access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.