@Kevin_Z  I have had exactly the same issue. I upgraded from the MR6400 to the Archer MR600 as I was only getting around 5MB/s download speeds in my rural location near Llangollen, North Wales. I transferred the SIM and the new router worked well with excellent data speeds for two days, before suddenly dropping the connection. Looking at the status lights it was obvious that it was no longer reading the SIM card.  I tried restoring factory settings, reinstalling the SIM and rebooting without success. I tried my phone SIM in the router as a test and that worked perfectly on a two hour test.

I contacted EE and arranged for a replacement SIM to be delivered. On testing the new SIM, the router made a very slow connection and then dropped. I logged on to the router and disconnected from EE. On reconnecting, I was suddenly getting download speeds in excess of 100MB/s! Without changing anything, I powered the router down, installed it in the working location and powered up. The router once again wouldn't read the SIM card no matter what I did. I took the SIM card out of the MR600, installed it in the old MR6400 and it works perfectly.
To me this seems like a firmware bug. I've seen on other threads that a fix is likely to be published soon. Any idea when?
Thanks Kevin.