@Raiden12345 

Hi，what is your current firmware version? and who is the ISP?

Besides, you mentioned that the same SIM card has no problem on mobile phone, may I know if you just insert it into the phone when MR6400 has the issue, or you've also run longer time on mobile phone to monitor the stability?

Regarding the LTE disconnecting issue on your MR6400, when it happens next time, please let us know the LED status, screenshots of Advanced->Status page and Advanced->Network->Internet page along with the system log file(please do not reboot the router before saving the logs).

By the way, how often usually does the issue happen? like once a day or several times a day?

The root cause might be different for different cases, so we have to do troubleshooting first to locate the issue, your understanding and cooperation will be highly appreciated!