Set up your Apple TV - Apple Support
Set up Dolby Atmos or surround sound on Apple TV with a sound bar or AV receiver - Apple Support
Play audio in Dolby Atmos or surround sound on your Apple TV - Apple Support
If you connect as
Apple TV box →[HDMI]→ AV receiver →[HDMI ARC/eARC]→ TV
then you would select the input device on the receiver, by its HDMI port numbering.
If you connect as
Apple TV box →[HDMI]→ TV →[HDMI ARC/eARC]→ AV receiver
then you would select the input device on the TV, by its HDMI port numbering.
Often, input selection can be automated through CEC. Activate this function on all HDMI devices in the setup.
For Apple TV: Settings﹥Remotes and Devices﹥Control TVs and Receivers﹥On
For the TV: See its manual. LG may refer to CEC using the ‘SimpLink’ trade name.
For the receiver: It is probably passive (reacting, not controlling) and always on for CEC. See its manual for details.
Verify that the status light is glowing continuously on the front of the Apple TV box. Then it has started up and is ready for use. If your Apple TV doesn't turn on or shows a black screen - Apple Support