DNS Rebinding Protection in router affecting the secure connections within the local network. They would be going to a url that looks like this https://xx-xx-xx-xx.xxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
and the dns lookup should translate it to your local server IP but if DNS Rebinding Protection is in place then the lookup gets rejected and a not found error is given
The web client app tried all secure routes and finds that it can get through on the xx-xx-xx-xx.xxxxxxxxxxxxxxxxxxx.plex.direct url for the wan public IP and so it uses it
You can find the routes to your server through the xml returned from
https://plex.tv/pms/resources.xml?includeHttps=1& X-Plex-Token=xxxxxxxxxxxx
and test each plex.direct url with nslookup - using the default dns and also try out google dns
The xxxxxxxxxxxxxxxx token you can find as advised here https://support.plex.tv/hc/en-us/articles/204059436-Finding-an-authentication-token-X-Plex-Token
There is a section on DNS Rebinding Protection on this support page
https://support.plex.tv/hc/en-us/articles/206225077-How-to-Use-Secure-Server-Connections
