Reset Apple TV 4th Gen and 4K that are stuck in Conference Mode
Connect Apple TV to a display
Plug in power
Unplug as soon as it finishes booting to whatever screen it boots to (usually the Conference Display screen)
Repeat until the Apple TV displays a “Recovery Mode” screen - about 3 cycles worked today, but go longer if needed
SOURCES FOR THIS TECHNIQUE:
This website gives the technique…
https://www.reddit.com/r/apple/comments/7y835c/anybody_know_how_to_restore_a_4k_apple_tv_it/
…and is referred to in the comments from this website, along with other potentially useful comments as well…
https://www.ifixit.com/Answers/View/462509/How+to+restore+Apple+TV+4K+(Without+USB-C)