R6020 loses internet connection after 30min

I just bought R6020 which loses internet connection totally after 30min. The internet LED turns amber.
The connection will not return automatically. I can reset the connection quickly from routers internet settings -> apply (without changing anything).
The firmware is updated to the latest available. WiFi and LAN connections between router and pc/laptops are working fine.

I have Alcatel standalone fiber optic modem (no router) and simply 5-port switch behind it. My older ASUS router is connected to the switch and it's still working fine without any issues. But now I have to add another router for my garage use.

So far I have tried at least the following:

DNS name service: changed Automatic -> specific IP Addresses
changed the MTU value 1500-> 1400
Tried to connect directly to modem output
Unplugged every other network components so there is no conflict

To me this seems like some sort of DNS issue. Any suggestions?