I’ll point you to https://forums.plex.tv/t/howto-use-plex-with-no-internet/383325 as a start about how to use Plex without Internet. A few minutes searching on the forums will no doubt bring you more information.
Plex does work without Internet but there are things you have to do before you lose Internet service. Using DLNA is also an option.
Yeah, I agree that Plex ought to make things easier in that regard but take the necessary steps now.
