Watch won’t pair

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) 

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

Make sure you have no VPN or VPN based adblocker on the iphone

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

Try again. 

Then I recommend reviewing the steps in the articles below 

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

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

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