Ensure that you have “Enable local network discovery (GDM)” checked.
Ensure that you have your LAN information in “LAN Networks”. Example: 192.168.1.0/255.255.255.0
Ensure that you have your LAN information in “List of IP addresses and networks that are allowed without auth”. Example: 192.168.1.0/255.255.255.0
You should now be able to access the server locally without having to sign into the Auth.Plex.TV by just going to your plex server. Example https://PLEX:32400/web/ (if your LAN server is named PLEX on your LAN)