I have both network and HDMI Tablo units that I use with Roku, FireTV, and Apple TV. I know the HDMI units don’t support Apple TV. When I first got the HDMI unit, it wouldn’t show up when scanning from Apple TV, I get that. Now it shows up when scanning, but won’t connect. Same for iOS devices. I wonder if that means an update to support these on the HDMI units is on the way?
I have a Quad HDMI unit and have no problems with my Apple TV 4k. Have you tried to delete the app and re-install? Just a thought.