Hello,
Does the connection recover after it is down?
Could you try configuring automatic connection restart? To achieve this, login to router's WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Edit existing default or add a new instance, enable it, set Action if no echo is received to Restart mobile connection. See if that helps.
Otherwise, 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.
Best regards,