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Plex Network Settings for Offline Access

The workaround is:

In your Plex server settings, go to Network, enable “Show Advanced”.

Near the bottom, find the textbox that says List of IP addresses and networks that are allowed without auth

In this field, enter the local IP address of any Plex client(s) you want to keep using if your internet (or the Plex cloud) is down.

A example: 192.168.0.50

Save the setting, done.