Plex Media Server version? 1.41.2 or later required.
Plex mobile app version? 2025.15.0 is current (release notes).
Does Settings → Remote Access show Plex is “Fully accessible outside your network”?
Restart Plex Media Server. Some people reported this was necessary after the remote access changes went into effect on April 29th. When you restart PMS it checks in with hosts at plex.tv and verifies you have a Plex Pass, can stream remotely, etc.