I've had TL-MR6400 for almost three months now--it was working perfectly but noticed today that the device frequently (every 5-10 minutes) disconnects and reconnects to the internet. It became worse as the day progressed.

I used the same SIM card (the one not working on the router) on a mobile phone, and it worked. Then I tried using another SIM card (from another carrier) on the router and it was working fine.

So this particular SIM card suddenly doesn't work on the router.

Does this sound like a router issue, a SIM card issue, or a carrier issue?

Router was updated to the latest firmware.