Thanks for the answer. I have the same problem. I want to run Jellyfin entirely on localhost, i.e. I don’t have a DNS registrar (or don’t know how to add a AAA record on YNH directly).
A similar post mentions a “hairpin” issue, that it by-passes the public domain to connect directly to the local server. I don’t know if that would help.
For reference, this question is also on github, with an answer suggesting deactivating SSO 
: Unable to connect to jellyfin from Android - port closed · Issue #183 · YunoHost-Apps/jellyfin_ynh · GitHub