Did you already have a look at the related troubleshooting support article?
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
There’s also a guide with some more details on how to troubleshoot remote access issues.
It seems you’ve already taken some of these steps. A good next step will be to test if your WAN IP and the specified public port are visible from outside your home network. You can test that using a service like csnyouseeme.org — this is also a good way to verify if the WAN IP seen by this service, your modem/router and Plex match.
