No "live tv" or "watch together" when using custom domain

Plex Media Server Remote Access
,
4

LiveTV, as in the free streaming, requires v4.38.2 or newer
Watch Together requires 4.44.1 or newer.

So you need to use app.plex.tv as the local web app is not updated to either of those yet. ( I do not have an ETA when it will be.)