How to set up Apple TV remote volume on iPhone
How do I set up volume control on iPhone for Apple TV remote
Apple TV 4K, tvOS 14
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When using the Apple TV remote app, the volume is controlled with the iPhone's physical volume buttons on the side. There's nothing that needs to be setup assuming your TV or receiver supports HDMI-CEC.
If it doesn't support HDMI-CEC then there's no way to control volume from the iPhone.
See here for details: If you can't control your TV or receiver with your Apple TV Remote - Apple Support