Apple Watch 5 Activity App Does Not Sync with iPhone 11

Make sure the iphone on iOS16.3 and the Watch is on OS9.3

Make sure your personal data is in the Health part of the Watch app, My Watch tab

Make sure your Health is turned on in iCloud settings

You should have the green phone symbol on the Watch's Control Panel if they are connected..

Airplane mode must be off.

Then force restart the Watch

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

 Then while it is restarting Force Restart the paired iphone   Force restart iPhone – Apple Support (UK)

Test it for a while. If it is still misbehaving... Unpair and set up again

 Unpair and erase your Apple Watch – Apple Support (UK)

and then

Set up your Apple Watch – Apple Support (UK)

That was part of my recommendations. Glad they helped.