I’m currently in a similar situation. I have recently moved and have been without internet for close to 50 days.
You have to edit some network settings to allow certain IPs to forgo authentication. You also need to adjust the time/date to be semi-accurate.
However, after working for a few weeks, I cannot get it to work now. I am totally clueless and it is beyond frustrating that it’s so difficult to work offline, which is the entire point of having Plex. I’ve gotten a Chromecast to work, but the Shield is pretty much useless now.