So I actually had this exact issue about a month ago.

Solution: You have to connect to chromecast through your phone app. However, the jellyfin app have to be installed through play store. If you install it through f-droid or similar it won’t work.

Took me about a month of troubleshooting to find that answer. :D

I dont remember the exact code but I solved a similar issue by waiting for network idle. There should be some info in the documentation for waitUntil

:)