You'll probably have a local domain for you're Jellyfin container, resolved via Pi-hole (something like
jellyfin.box). And when casting from the Jellyfin app to a Chromecast, the screen is stuck on the Jellyfin logo.
This is because the Chromecast can't resolve
jellyfin.box. Chromecast has its DNS-servers hard coded to Google's DNS-servers.
To resolve this i rerouted
220.127.116.11 in my router to my local DNS server (Pi-hole).
In FRITZ!Box you do this via
Home Network >
Network Settings >
Static Routing Table and open
IPv4 Routes. There you can add 2 static routes.