Hey there! Sorry to hear you are having trouble with your Apple watch. Of course, we can fix this with a simple factory reset (Notice. This will erase all data on your Apple Watch.) I also do recommend that you check for updates once you pair to your new phone. You can see how to do this here
In order to perform a factory reset, just follow the steps below:
- Open the Settings app on your Apple Watch.
- Go to General > Reset, tap Erase All Content and Settings, then enter your passcode.
- If you have an Apple Watch with a cellular plan, you’re offered two options—Erase All and Erase All & Keep Plan. To completely erase your Apple Watch, choose Erase All. If you want to erase and then restore it with your cellular plan in place, choose Erase All & Keep Plan.
You can also open the Apple Watch app on your iPhone, tap My Watch, go to General > Reset, then tap Erase Apple Watch Content and Settings.
If you can’t access the Settings app on your Apple Watch because you’ve forgotten your passcode, put your Apple Watch on its charger, then press and hold the side button until you see Power Off. Press and hold the Digital Crown, then tap Reset.
After the reset finishes and your Apple Watch restarts, you need to pair your Apple Watch with your iPhone again—open the Apple Watch app on your iPhone, then follow the instructions shown on your iPhone and Apple Watch.
Taken from: https://support.apple.com/guide/watch/erase-apple-watch-apd4ad3571d9/watchos#:~:text=Open%20the%20Settings%20app%20on,and%20Erase%20All%20%26%20Keep%20Plan.