My Harmony Hub recently stopped powering on my Apple TV4k. I tried several things (removing Apple TV, Reworking Watching Apple TV activity, etc.) to no avail. I finally went into the Edit Activity function and added a custom command to the startup sequence for the Watching Apple TV Activity. I told the Harmony Hub to send a "Menu" command to the Apple TV when starting the Watch Apple TV Activity. This is the same as pressing the "Menu" button on the Apple TV remote. It worked. Now my TV, Apple TV, and receiver all turn on and operate as they always have.