Try some of the trouble shooting steps at this post (Plex inaccessible from same local network on web app through browser - #4 by kesawi) but apply them to the remote/WAN IP instead rather than a local IP.
Use a port checking tool (e.g. Open Port Check Tool - Test Port Forwarding on Your Router) to check that your remote port and IP are accessible.
Report back your actually findings and test results, not just that you’ve tried them and everything shows okayas that doesn’t give us anything to go on.
