Are you using the local IP address of your server machine during the procedure? You must NOT use a domain name.
Is the IP range of your local network in one of the designated ranges for “private” use?
Try this registry file instead:
[-HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server\PlexOnlineHome]
[-HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server\PlexOnlineMail]
[-HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server\PlexOnlineUsername]
[-HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server\PlexOnlineToken]
[-HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server\PreferredNetworkInterface]
[HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server]
"PublishServerOnPlexOnlineKey"=dword:00000001
"ManualPortMappingMode"=dword:00000000
"LogVerbose"=dword:00000000
Rest like in the linked article.
