This problem affects only obsolete Turris OS 3.x release
If you use CZ.NIC TLS DNS forwarder (ODVR) like this on your router
you would encounter DNS resolving issues and both of your router and local network will be probably unusable.
Root cause of this issue is in older GnuTLS library that is present in Turris OS 3.x and there is no such problem in current and stable Turris OS 5.x release.
To fix that, I recommend to exchange DNS forwarder to other servers like Cloudflare or Quad9 or disable forwarding at all. The best option, however, is to upgrade your router to Turris OS 5.
We would post some update here if anything new about this issue will appear. Further details are available in GitLab issue at turris/os/packages#799.