The deco seems to prefer connecting to a 2.4 GHZ network on the base Deco vs. a closer Deco where 5.0 is in range.
Since 2.4 goes further than 5.0 the AppleTV cannot connect to the 5.0 on the base, but it connects to the 2.4. By turning off the 2.4 network, I got the AppleTV to connect to the 5.0 network using a Deco in the same room. I even turned off the mesh technology on that device using the app, since the AppleTV does not move.
Then I turned the 2.4 network back on, since I have devices like Sonos, Ring, Roomba that only work on 2.4. Next time I went to use the AppleTV it connected to a different Deco, back to the main one, on the 2.4 network. It seems like the button to run off Mesh Technology does not actually prevent it from going to a different Deco device.