More details are needed to provide any suggestions.
What do you mean by router stops sending internet? Does the WiFI connectivity stop working, or mobile data connection is lost within the router after disconnecting LAN cable?
Next, I would like you to do the following: recreate the basic setup, insert the SIM card, restart the device, then wait till it reboots and connects to mobile network. Connect your computer via LAN port, disconnect from the route'r LAN port, make sure the device stops sending internet. Then connect to the router again, login to the web interface, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. Attach the downloaded file by editing your question.