To force-quit an app on the ATV, double-click the ‘screen’ button. This should bring up the tiles for all apps open in the background (similar to an iPhone or iPad). Use the trackpad to scroll through the tiles until you find the Zwift app, and then swipe up on the track pad (or just swipe up to close all open apps). This should free up your HRM to be detectable by other apps.
@Nigel_Tufnel you are awesome! All sorted now 
I have a Bkool Pro 1 trainer and trying to run it on a apple tv. I can’t find my device under ‘controllable’.
Have tried to download the BkoolTool App to update the firmware but it just keep searching.
Can you help me with any guidance for how I can get it to work?
Are there any USB ANT+ hubs that can be hooked up to Apple TV?
Is it possible to use Garmin Edge as Ant+ hub?
For example, I have Tacx that is only good at resistance simulation. Power, Cadence are coming from Assioma pedals. I also have a speed sensor that seems to be pure Ant+. Garmin HRM is BLE/Ant+.
According to this link (which I have not verified elsewhere), when pairing via Bluetooth thus trainer cannot be controllable.
There is no support for ANT+ in tvOS so you would need a device that bridges ANT+ to Bluetooth. This is one that I’ve heard about (haven’t used it).
Hello, I have a new Apple TV and i connect it to the Garmin bike directly whit the bluetooth but I experience a lot of disconnections, is it possible to connect the bike to the ant+ which is more stable, unit and from there broadcast via bluetooth to the Apple TV?
If you are having a lot of disconnects, there is likely something wrong with your setup, or there is interference in the area. Can you provide a complete rundown of your setup?