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. After confirming the port forward is properly working and configured in the router and Plex (as per MovieFan‘s response)… it might be a good step to confirm if your ISP is assigning a publicly routable IP address to your router (and if that public IP address matches as seen in the Plex Remote Access Settings, your router‘s WAN IP and that seen by your external port checker).
