My watch won’t connect

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

Then;

Setup your Watch to a new iPhone

Select the appropriate watchOS from the dropdown menu in this guide --> Set up and pair your Apple Watch with iPhone – Apple Support (UK)