It has to be setup for local network streaming (and requires a network connection to do so the first time). Plex is far from perfect but its pretty much the best local media service I've found, so I'm not too mad at some of its idiosyncrasies.

Getting plex pass is a must if you use it regularly in order to support hardware encoding.

Plex works without Internet just fine, but very annoyingly to enable local streaming without Internet, you need internet.

Had to enable hot spot on my phone briefly when I had an internet outage. Annoying but now just works.