Restrict content using managed users - without access to the world wide web

Server Version#: 1.24.4.5081
Player Version#: 8.23.1.28053
Server running on#: QNAP TS-451D2

Dear Plex community,

A few weeks ago I bought the Plex Lifetime Pass. The Plex Server is running on my QNAP NAS. But my NAS (and Plex Server) are only available in my local network because the access to the world wide web is blocked from the router.
Now I’m looking for a poosibility to restrict the content to the Clients in the local network while the server has no access to the world wide web. I created new managed users to restrict the content. The problem is that as long as my NAS/Plex Server has no access to the world wide web the NAS can’t be reached by the client. If I allow the access to the world wide web the Clients can find the Plex Server…
It worked as well if I add the IP range of my local network to the List where no authentication is needed. Then the Clients are able to find the Plex Server without a connection to the world wide web but the complete content is available and no restrictions are working.

Can you help me to find a way, how to restrict the access to the Plex Content in a local network without access to the world wide web for the Plex Server.

Thank you so much in advance for your support.