The primary (only?) reason Plex requires internet access is authentication. This is used to prevent just any Joe on the internet from punching in your house’s IP address and being able to view your Plex server contents. You see this if you use Jellyfin, you must set up your own authentication scheme for external network access. So, if you have authentication REQUIRED on the server and client side (this is the default), then when the internet goes down you will be unable to authenticate the client, and thus whether you are authorized to access the server.
There are some guides on how to set this up. One good one knocking around is this one, which explains how to set up Plex for internet-less connections. Reviewing it, I am a bit disappointed in some of the client devices that won’t work even with certain settings enabled (like my FireTV), but there are still some good clients that work anyway. In particular, they walk through enabling a Roku to view the Plex server’s content.
Note, one thing Plex authentication does is it also returns to clients the LOCATION of the server: The local network IP address. Without authenticating, you must enter it in yourself. This is covered in the article I linked.
I can summarize the steps you need to take, but I’m going more on memory/assumptions about the setup, so I’d rather just ask you to refer to the link I posted up above. If you have more questions about particular steps, feel free to reply with those here (or there), and I’ll see if I can help.
Some of the things lost when using Plex offline:
- Fast User switching. If you use multiple profiles on the account to keep track of separate watch states for multiple users, this will be unavailable. Technically, when you bypass authentication, you log in as the default Admin account itself, not any of the managed users. If you do not use managed accounts, then this isn’t a loss.
- Online media. If you have no internet, then obviously the free “Movies and Shows on Plex” won’t work.
- DVR guides. With no internet, the Plex server cannot populate the DVR guide data, so after some time (week?) it will run out and scheduling breaks. You ought to still be able to view the channels (I hope/assume), but I haven’t tested it.
- Plex pass features? I would think that after some time, your server might not be able to verify if you have an active Plex Pass subscription. If I designed the app, I’d have the Pass features eventually disable (intro skip, DVR, etc) if the server has not authenticated recently. But I am not so sure about this section.
