did you tick Manually Specify Port in server settings for remote access and enter 32400 in the box?
could be the issues that have just been resolved in beta 1.14.0.5468
See release note Plex Media Server - #239 by drzoidberg33
- Automatic port mapping could fail with some older routers (#9335)
- Fix for Server Unreachable and Remote Access issues (#9228)
It is currently released as beta - don’t know when it would be public
