I’ve had a similar problem to yours (on mac however) I’ve rooted the issue to the checkpoint VPN that had installed security policies that were blocking incoming/outgoing connections to plex as well as remote access and other services. Having uninstalled the checkpoint VPN using the checkpoint uninstaller (making sure it has been uninstalled cleanly) and restarting, remote access to plex has been working perfectly now.
It might be worth adding a section to the plex help topics to cover VPN’s that install these policies as they seem to be quite a popular setup (albeit shoddy) with corporate laptops.
