Plex is designed to have the Internet always available. The lack of local authorization causes problems when the Internet is unavailable.
See [HowTo] Use Plex with No Internet for information that can help. The document mentions making entries in List of IP addresses and networks that are allowed without auth. Be aware that any such devices have server owner level access. They can see all libraries. Using Plex Web / Plex Desktop, they can reach and change server settings.
Using Plex without the Internet was raised in the recent Pro Week AMA discussions. See this post: Pro Week ‘25: Fireside in the Forum AMA - #46 by McWanke.
