Most likely culprit: “DNS rebinding protection” in your router/DNS resolver.
-
Don’t set ‘Secure Connections’ to “required”
under Settings - Server - Network -
Set your preferred DNS server to Google 8.8.8.8 and/or 8.8.4.4
-
See if you can apply one of the fixes in here: https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections#toc-4