Under paired devices my polar H9
HRM shows and connects in RUN
mode but when selecting RIDE it
is not found??
I have the same problem with my coros HRM. I found if I connect the Coros to my Garmin watch, and start an indoor ride. Zwift is able to find my watch (HRM). After the ride is over, I just bin my Garmin ride. My Wahoo HRM connects to run or ride 
I also found that in Zwift HR is approx. 15 beats higher than other devices (eg. treadmill display) same time. Zwift is not accurate.
Zwift just displays data from your HRM, nothing else.
So Zwift read HRM signal somehow wrong. Like I said, the HR is different same time on two different devices. Both read same HRM same time. I am so old that I can feel diffrence between 135bpm and 150 bpm. With that higher bpm I almost should be in emergency room 