Cant cast local media via chromecast

So heres my problem.

When im out and about, id like to cast local media via the chromecast to my headrest monitors.

Set up:
Tablet
Chromecast
Headrest monitors

Config:
Tablet is hot spot
Chromecast tethered to tablet
Plex on tablet
Choose local content
Cast to chromecast
Works, i get the plex logo
Choose media, press play
Plex just spins as if its loading, no time out, no cant play media, nothing, just spins.

Now, if i connect to my server, everything works along with my data usage going up, lol.

Id like to play local content to spare my data usage.

Any thoughts here?

You cannot currently cast local content. The app can’t transcode videos and there is no guarantee that the file will be supported on the CC, so this will not work.