Garmin dual heart rate monitor connection issues

My set up is Iphone ==> Companion App==> Apple TV. I am using Taxc Neo. With one of my phones the Garmin dual heart rate monitor does not connect via Companion app. But with my other phone it does. OK - I am using that phone for the moment but would prefer to use the other one (iPhone X) It worked very well for a long time but suddenly it stopped connecting to companion. The other blue tooth sensors connect without problem (Trainer etc.)

I have tried to remove the app and re-install it, but nothing changes.

Any ideas anyone?

Thanks / Mikael