Why does my Apple TV 4K disappear from the app?

The short answer

AirPlay and HomeKit rely on background discovery messages that some routers filter. The gadget is usually still online. What's broken is your phone's ability to see it, and that is a network setting rather than a fault with the device.

What's happening underneath

Significant for casting.

Source: Manufacturer documentation

Try these, in this order

• • Put the phone and the Apple TV on the same Wi-Fi name.
• • Turn off any isolation setting on the router so the two can see each other.
• • Plug the Apple TV into the router with a cable if one is within reach.
• • Check the signal where the gadget actually lives, not where you're standing.
• • Move the router off the floor and out of any cupboard before buying anything.
• • Put the gadget and your phone on the same Wi-Fi name.
• • Restart the gadget before the router — it is quicker and it fixes more.

Setting it up again properly

• • Stand next to your router while you set it up, then move the gadget to where it lives.
• • Put your phone on the 2.4 GHz network name for the setup, or use one name for both bands so the phone can follow.
• • If your router splits bands into two names, turn the 5 GHz one off for five minutes rather than fighting it.
• • Turn mobile data off on your phone during setup — some apps quietly use it and never find the gadget.

When it's worth spending money

Only if 4K streams buffer with a strong signal — then it's your broadband speed, not the box.