Hi
As your watch is not showing under devices in iCloud, try the following steps:
If you have not yet done so, unpair the watch via the Watch app on your iPhone (even though the watch is not available):
- On your iPhone, in the Watch app, go to: My Watch (tab) > My Watch > Apple Watch: if the button is available, tap on Unpair Apple Watch.
Then ask the new owner to try these steps:
- On their iPhone: close the Watch app along with all other open apps (double-press on the Home button, swipe up on each of the app previews).
- Restart both devices, turning both off together and then restarting the iPhone first:
- To turn the watch off: press and hold the side button until they see the Power Off slider; drag the slider to turn off.
- To turn the watch back on: press and hold the side button until they see the Apple logo.
- Then re-try setting it up.
It is not advisable to provide the new owner with your Apple ID credentials, even if you change your password again immediately afterwards.
If the watch remains locked, try contacting Apple Support again and explain the situation, making clear that you are the original owner. They may then be able to assist you further:
https://www.apple.com/support/watch/contact/