We had difficulty finding the Polar H10 heart rate monitor in Polar Flow, Polar Beats and our favourite app SportsTracker Pro (no longer supported).
The short answer to our problem:
Do not connect the H10 to the phone’s Bluetooth. Let the app discover it. If it’s connected in the phone’s list of Bluetooth devices, unpair it.
We’re used to pairing devices to Bluetooth like headphones, portable speakers, car head units, etc. Did not realise that Polar heart rate monitors worked that way.
We also came from the Polar iWL (wear link). It was an older device that connected in the traditional manner. Subtle difference, but important.