@LPC 

When the problem happens, are you able to login the web interface of the MR600? If so, please send me a screenshot of Advanced->Status page when losing internet and also send me a system log saved on Advanced->System Tools->System Log.

Besides, please also check network type and band on Status page when internet works fine and also when losing internet, if the band is different, the problem might be caused by band switching, please try to select 4G band manually to see if it will be more stable:

https://www.tp-link.com/en/support/faq/3099/

In addition, since it was working stable before, do you have any other 4G device available to test for comparison? which will help to confirm if it is router issue or ISP/base station issue.

By the way, when the issue happens, will it recover automatically or you have to reboot the MR600 to get internet again?