What to do if my Apple Watch won’t connect to my phone

First check you have no other sports watches like Fitbit that are paired to your iPhone

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 Force Restart the paired iphone 

Force restart iPhone – Apple Support (UK)

Try again to pair the Watch.

Then in the iphone, Settings, General, Transfer or reset iphone, Reset, Reset network settings.

reconnect the iphone to Wi-Fi

Make sure you have no VPN or VPN based adblocker in the iphone in Settings, General, VPN & Device Management.

Then reset the router to factory settings which usually involves a paperclip on a little hole.

Try again to pair the Watch.

Then I recommend reviewing the steps in the articles below

How to pair an Apple Watch with your iPhone

Click: How to pair your Apple Watch with a new iPhone - Apple Support or

Can’t pair an Apple Watch with your iPhone

Click: If you can't pair an Apple Watch with your iPhone - Apple Support or

Apple Watch not paired with your iPhone

Click: If your Apple Watch isn't connected or paired with your iPhone - Apple Support