Maybe you need to get a new HDMI cable? It's (obviously) more than two channels. "Spatial Audio" - where the soundscape moves when you move your head - only applies to headphones. When I looked at pics for the back of that receiver, I see it has eARC support - which typically suggests it has either HDMI 2.1 or 2.0a+ features. Me, I'd make sure I'm using an 8k 48Gbps HDMI 2.1 cable to make sure that the Apple TV sees sufficient bandwidth to send the Atmos signal.
As the receiver came out in 2019, I'd also check to see if there was a firmware update available for it from the manufacturer.