You can reset your watch without the iPhone, have a look at the steps mentioned in this article:
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- Put your Apple Watch on its charger, then press and hold the side button until the sliders appear.
- Press and hold the Digital Crown to show the Erase all content and settings screen.
- Tap Reset, then tap Reset again to confirm.
- When the process finishes, set up your watch again. When asked, restore from a backup.
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copied from: If you forget your Apple Watch passcode - Apple Support
But after that, the Activation lock will still be enabled:
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If you can't access your iPhone or Apple Watch, or if your Apple Watch isn't paired with your iPhone and you need to turn off Activation Lock:
- Go to iCloud.com and sign in with your Apple ID.
- From the Quick Access Menu in the top right, select Find My. You might be required to sign in.
- Select All Devices, then click your Apple Watch.
- Click Erase Apple Watch. Select Next until the device is erased.
- Click the Remove button next to your Apple Watch.
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copied from: About Activation Lock on your Apple Watch - Apple Support