DNS issue
or
DNS rebinding protection
- set the DNS server to the one provided by Google (in your router) (8.8.8.8 and/or 8.8.4.4)
- disable DNS rebinding protection for the domain
plex.direct(also in the settings of your router)
https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections/