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.
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.
Agreed - need to treat it as adding a new device - problem solved