After restarting the phone, unistalling the app and reinstalling the app the watch refused to pair with the phone.
I restarted the watch by pressing and holding the light button (top left) for 15 seconds. Then pressed it once. Reconnected, repaired it started to work.
After unistalling the app, restarting the phone, reinstalling, it refused to pair so held the light button (top left) for 15 seconds, powered down, powered back up with same button. Reconnected, repaired, all worked.
I had problems pairing with a new iPhone 11. In addition to the suggestions here: I ran a software up date on the watch. Unpaired all sensors, HRM, speed and cadence. It then worked. No sure what made it work but could be one of those actions.