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.
I recommend you start by verifying that your port forward is visible/accessible from outside your home network (e.g. using a service like canyouseeme.org). You should also verify if the public IP address seen from the outside matches that in the Plex remote access settings and your router’s WAN IP.