CEC problem with 3rd Gen 4K

Verify the setting on Apple TV: Settings﹥Remotes and Devices﹥Control TVs and Receivers﹥On.

If you can’t set it to On, then try disconnecting the HDMI cable on both devices. Else try restarting all HDMI devices in the setup.

Changing the connected device may have reset the CEC setting on the projector. Verify that CEC is still set as active.

There are no further options available.

the Apple TV is not responding in any way

Apple TV would be the commanding device, not the responding device.

Benq projector and it expects to be turned on by another CEC device

If you say so. That does not apply to all their models: e.g. HT3550, HT3550i, TK850i do not.

What is this model projector?

Apple TV (or any active CEC device) can send commands (suggestions) to wake or go to standby to other devices over HDMI, but it is up to those other HDMI devices (and their settings) if they will listen to that.

CEC commands go over a dedicated pin. A faulty or dirty port could hinder that. A faulty wire or faulty or dirty pin (#13-signal, #17-ground) on the plug could hinder that. If you can, test alternate HDMI cables.

This tip worked for me. I have had LG HU85LA and Apple TV 3gen working flawlessly for about 3 years. Suddenly LG stopped sleeping or waking via Apple TV but fine in every other way. Cleaning HDMI cable connectors with contact cleaner on each end fixed the issue for me. Copper pins on cable must have oxidized over time.

Came here with the exact same issue. Switch from 2019 shield pro to AT4k 2nd gen. Same cables, remote can control Yamaha sound bar, but won't give me the option to turn on the Control Home Theatre/TV option. CEC is active on my Samsung TV, but no luck. Worked on my shield, but a swap of devices using same cables had no success with the new device.

Have you found a solution? I ordered new cables but doubtful that would work.

TIA