I don’t want to prevent access I want to force or not force authentication.
IE DLNA clients need no authentication, my Roku supports plex pass and if I want the web UI to go thought my ssl reverse proxy I need it to have forced authentication because Plex sees it as a. Local resource. I work around that with http auth right now but it limits things.
Under plex settings / network / advanced there is an option listing of allowed networks without auth. You could just add 192.168.1.x/255.255.255.255 to that list with the IP of your DLNA device, provided it didn't change IP's very often. This combined with require local network auth acts as a kind of white list.