the actual problem here is that OPs network is not configured correctly and that Plex detects that the physical local client is actually accessing the server from a totally other network.

Fairly common when you use docker to run Plex and have the container run in bridge mode. This will put the container in the docker network that will then be different to your local network.

Plex determines if a stream is local or remote based on the network so when your container is in bridge mode, the physical local client will be a remote connection because of the different networks.

And since remote streaming requires Plex pass since end of April, you will see this.

LAN networks is only available for plex pass users