This will only play to HomePods while using the Apple TV - choose your HomePods in ATV Settings The default audio setting for Apple TV requires the ATV 4k model and must be assigned to the same room but you can otherwise AirPlay to HomePods with other ATV model. See links provided below.
- Set it up to the other HDMI port:
Plug your Apple TV into power and connect it to your TV with an HDMI cable. To watch movies in 4K HDR on Apple TV 4K, make sure that you’re using an HDMI 2.0 or later cable, and have a TV that supports 4K, HDR, or both. Then turn on your TV and select the HDMI input that your Apple TV is connected to.
If you have an Apple TV with an Ethernet port, and you don't want to connect to Wi-Fi during setup, you can connect your Apple TV to your router with an Ethernet cable.
AirPlay - Apple
Set up home theater audio with HomePod mini or HomePod and Apple TV 4K - Apple Support
Use Apple TV to play audio throughout your home - Apple Support
More: Adjust video and audio settings on Apple TV - Apple Support