trouble pairing Polar H9 heart rate monitor chest strap to Apple watch and iphone. Sometimes connects, sometimes not. When connected, watch heart rate lags behind iphone by several seconds. Not acceptable when doing interval training.

I bought a Polar H9 heart rate monitor to be used for interval training. I want my heart rate to be transmitted to my Apple Watch. When I pair the monitor to the watch, and use the Polar Beat app, I get no heart rate ( and I am not dead). When I pair the monitor to my iphone and use the Polar Beat app on the watch, the heart rate on the watch lags behind the iphone by several seconds, which defeats the purpose for which I bought the monitor. Today, I cannot connect the monitor to my phone: it says "Connection Unsuccessful". I powered off my phone, and still cannot connect phone to monitor.

Can anyone help? My phone is an iphone 11, and the Apple Watch is Series 7.