I have the same issue.
The repo seems to be down or not reachable. Nor ReForis, nor LuCi, no pkgupdate, nor opkg update, work to update Turris. Internet access to any other website is definitely accessible. Even ping to the repository is replied, however address translated from repo.turris.cz to proxy.turris.cz.
root@turris:~# ping repo.turris.cz
PING proxy.turris.cz (217.31.192.69) 56(84) bytes of data.
64 bytes from 217.31.192.69 (217.31.192.69): icmp_req=1 ttl=57 time=31.9 ms
64 bytes from 217.31.192.69 (217.31.192.69): icmp_req=2 ttl=57 time=32.2 ms
^C64 bytes from 217.31.192.69 (217.31.192.69): icmp_req=3 ttl=57 time=32.2 ms
--- proxy.turris.cz ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 10072ms
rtt min/avg/max/mdev = 31.938/32.127/32.230/0.133 ms
I am with Turris 5.1.10, and from the /usr/share/updater/updater-log, the last upgrade happened on 5/03/2021.