I vaguely recall that Chromecast will always attempt to use Google DNS or something? On my router I set it up to intercept all DNS requests and respond to them. Can your router do that?
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)