Hitherto been using iPhone app with no problems. Use Kinetic’s Road Machine for power and cadence (although I ignore because not very accuracy) and Apple Watch for HR
Bought Apple TV and tried using instead. I can connect power source and disconnect cadence and connect Apple Watch, but only ever get connected but no signal for HR. Tried using companion app BLE, but never get option when connecting sources to use it.
Natively, Apple TV has a hard limit of 3 simultaneous Bluetooth connections. The ATV remote uses one of them, leaving only two open to pair with your trainer. That typically means you don’t have an open connection for your HRM.
In cases where your devices (HRM or trainer) broadcast with ANT+, you can use Zwift Companion on a smartphone to bridge that ANT+ connection to the AppleTV. Here is some info on that.
AFAIK, Apple Watch is Bluetooth only. Try pairing that via native Blueooth to the Apple TV, and pair the trainer over Zwift Companion.
Also - be aware that Zwift Companion and the Zwift game app “see” each other over WiFi. Your ATV and smartphone must both be connected to the same WiFi hotspot.