You’ll need to provide some more details to get a solid idea.
Remote access involves a lot of parties from your ISP who could be blocking certain communication / ports or lock you in a CGNAT… to a bad router configuration (especially if there’s multiple components involved through which traffic needs to hop in order to “get out”)… down to (last not least) the server itself (potential hiccups with the OS or Plex configuration).
It’s usually a good starting point to go through the basic troubleshooting:
