Cast Plex locally w/o internet

My internet connection is out and I'm trying to cast from my Plex (iOS) to the Chromecast.

Chromecast is connected to same wifi network as phone and Plex server. Chromecast reads 'Reconnect me' (Connected to wifi network but not internet). Phone is able to connect to Plex over wifi and get content. When I try to cast with phone Plex app, Chromecast starts to load (the screen goes black), waits for about 10 seconds, then dies to a screen that reads 'Oops, not supposed to be here'.

Shouldn't I be able to cast purely locally with this setup? Has anyone encountered a similar issue?

EDIT: Enable local network discovery is turned on for the Plex server