I've tied everything to no avail. Is it because Plex is behind a LAN Extender?

[tom80H]

There should be no issue if the 2nd router is in bridge mode and not establishing its own subnet.

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.

The Basics of Remote Access Troubleshooting

When it comes to remote access, there’s a lot of different factors which can impact your setup. Some are under your own control, others not so much. First off… there’s a dedicated support article on troubleshooting your remote access. I strongly recommend you have a look at that. Find below some additional remarks on how to troubleshoot your remote access setup. Review your internet access / remote access configuration It’s good practice to review if Plex gets the correct public IP address…