Unless I am doing something wrong you have to be signed in on the Plex app on your tv in order to cast to it from Plex on another device. If I am signed in on the Plex app on my phone it would be good if I could cast to a tv without having to sign in there too.
This would be useful if you wanted to show an episode of something on a tv that isn’t yours (work place, friend’s house, etc).
