Hello,
The router has a feature, which allows to configure the device to automatically perform certain actions on mobile data connection loss, such as modem reboot.
To configure, login to router's WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Edit existing or add a new instance, set Action if no echo is received to Modem reboot. Save the settings.
Alternatively, if you are not doing it already, if the router is stationary and the direction of the cell tower is known, I would suggest to try deploying a directional antenna. This should boost signal quality and reliability.
Best regards,