I too have the same problem, the router was working fine up to the beginning of this Month (exactly) when it suddenly started failing to resolve repo.turris.cz
I run pi-hole which also serves my DNS & DHCP as a container on the router, WAN Adv settings have the Pi-hole as the DNS address.
It’s not an internet outage, all my devices on the network resolve repo.turris.cz the router fails to resolve the address however.
Where can I force the router to use the pi-hole for it’s internal DNS query? (as I suspect this is the problem as it has a static LAN address.)
I’ve worked around this for now by adding 184.108.40.206 repo.turris.cz to my hosts file.