I recently upgraded to 1.9.2 from 1.5.7 and now all my attempts to connect to my Ubuntu server from the app.plex.tv domain result in a warning indicating that only indirect connection is available.
I have verified that the port 32400 is open and forwarding properly to the server. I can even work around the issue by using navigating directly to the server using my domain name on port 32400. This isn't ideal as I have lots of users and I don't want to require them to use my domain name to work around this issue.
It appears that at least a couple of other users ran into this issue on ubuntu but their resolution was that it simple "went away". https://www.reddit.com/r/PleX/comments/6z2iop/plex_says_not_available_outside_your_network_but/