I like shitting on Plex but you absolutely can stream entirely locally without internet.

There is a server side setting you need to switch off. If I remember correctly it deals with the way you sign into plex.

I won't doubt that. I've just had Plex for at least 6 years and never had the complete inability to stream directly when the internet is down. It has always fallen back to local streams in my experience (when I had Comcast, this was a frequent occurance and would have otherwise resulted in me returning to emby).