How many devices connected:

3-4 ethernet

2-4 wifi

Is there any other setting or feature in your network that may cause this restart issue, or is there any special operation/activity in your network that causes the C1200 to restart?

I'm not sure what could cause this. The network contains few smart tv, few windows pc + few linux pc + phones. I don't really do much more than network file sharing and ssh over local network

Please check if the C1200 is hot when it reboots automatically, put the router in a well-ventilated location or mount it vertically on a stand that permits the free flow of air, and use a fan to cool the router, a regular fan or USB fan is fine.

The router is warm  like 40-50 C (on the bottom plastic) when I have tried it. Is the passive cooling for the router not good enough for room temperature of 22 C where it is sitting on hard surface? Is it normal that the router will just overheat under I would say "normal operations"?

Try to reset it to factory defaults by holding the reset button on it until all the LEDs go off then on, reconfigure it from scratch, then monitor again.

I've tried this before and it did not work. If none of the above helps resolve it, please try the below beta firmware on your C1200 and confirm if the auto-reboot issue will be resolved: Archer C1200(EU)_V2: https://static.tp-link.com/2020/202001/20200103/Archer C1200(EU)_V2_180308(Beta).zip

I will try to upgrade to the beta fw