As an alternative to rebooting the device completely, you could only reboot its modem on events of mobile disconnection, which is faster and leaves other device services up and running.
To configure automatic reboots, login to router's WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Edit existing default or create a new instance. In its settings, enable it, set Action if no echo is received option to Modem reboot, adjust the rest of the settings to your needs or leave them as default.
As for the mobile connection instability, it is difficult to estimate without some logs from the device. For this, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.