Your AppleTV only accepts Bluetooth signals. Your strap only transmits ant+. I'm not sure if the Fenix 3HR transmits HR at all, but if it does transmit HR, it will only be in ant+. I believe the FR945 and Fenix 6 were the first Garmin watches to transmit in both ant+ and BT. If you want to use your existing strap, you could purchase a CABLE device...https://npe-inc.com/cableinfo/. It will convert the signal from your strap from ant+ to BT, so your Apple TV can receive it. Or just buy a new strap that transmits in both BT and ant+, which is about the same price as a CABLE device.

Thanks for that explanation. I guess that what I get for having “old” technology.

I wouldn't say you are using "old technology." True, the Fenix 3HR is kind of long in the tooth, but ant+ is in many ways superior to BT. With BT and AppleTV, you are limited to a certain number of devices you can connect. Ant+ eliminates all those restrictions and probably isn't going away anytime soon.

Pick up this or something similar and you will be good to go.
https://www.rei.com/...BwE&gclsrc=aw.ds

Like other have said Ant+ is still very much a good technology it's just that Apple TV doesn't have it.

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