My Polar HRM 10 has worked great with Zwift. But now connected but no signal. Have updated Apple TV and put new battery in… still no signal. Same thing happens if I try to use Garmen watch…
If it happens with both devices then it sounds like the Apple TV is the issue rather than the Polar HRM. I’ve not got an AppleTV (i use an ipad) but what has worked for me when getting odd behaviour is to completely remove the app and then reinstall it. Its annoying as hell, but its fixed a couple of problems for me now. Do you want to give that a go and see if it helps? As a warning though, you will obviously lose some cached local data such as login details.
Is it possible that your HRM is connected to something else?