The Custom server access URL field doesn’t accept a bare IP address typed directly into the field.
It does accept this format: http://198.51.100.51:[port].
If an address is entered with that format, Plex will also automatically register the URI https://198-51-100-51.[guid].plex.direct:[port], which will automatically enable SSL using a Plex-provided certificate … no custom domain name or additional certificate required.
When using a Custom server access URL, it may be appropriate to disable Remote Access. If this is not disabled, the server will continue to discover and (also) register the incorrect IP address.
