I’ve noticed that recently Channels will just stop connecting to any stations occasionally and just never get past loading any channel. Manually closing and reopening the app (not just exiting and coming back) doesn’t change anything. I’m able to stream to other apps & devices. Restarting the AppleTV fixes the problem every time.
HDHomerun Extend (transcoding enabled) / ATV4 tvOS11 / Wifi / Also have Plex DVR on the network (only using 1 of the 2 tuners at most).