Assuming it is not currently paired, erase the watch with Setings, general, Reset, Erase all content and settings.
Then use a computer to update your iPhone to iOS16
Learn how to update your iPhone via recent Macs here:-->
Update your iPhone, iPad or iPod touch with the Finder – Apple Support (UK)
or older Macs and PC here:-->
Update your iPhone, iPad or iPod touch with iTunes – Apple Support (UK)
Then check the following to ensure Wi-Fi is working
· Reset the router to factory settings (menu or paper clip) – even if all other devices connect OK!
· On iphone Settings, General, Transfer iphone or reset, reset, reset network settings
· If you have a VPN you are strongly recommended to uninstall it and its profile in Settings, General, VPN and device management
· Try settings, Privacy & Security, Location Services, Systems Services(at the bottom), turn off Networking & Wireless.
· Try turning off Private Address in the Wi-Fi connection’s settings (i)
Then click here :- Set up your Apple Watch