I have seen thie question asked a lot lately so I thought that I would create this topic in Smartthings instead of posting the Home Assistant link.
First, remove the device from SmartThings doing exclude or if that doesn’t work then force remove. Exclude is preferred, but sometimes it is what it is.
Then, reset the device:
1 - Press the top of the paddle switch until it clicks and hold for at least 7 seconds, then release.
2 - Press the bottom of the paddle switch until it turns a solid amber, then blinks red once. Then, release.
Now your device is reset once you see the single red blink.
Add it to SmartThings by:
Press up on the paddle and hold for about 7 seconds
Press up on the paddle once for a short period.
Press down on the paddle until it turns solid amber. (5-7 seconds)
In SmartThings go to add device (plus sign at top) then scan nearby.
Within a few seconds, the device should display in the list of devices in the room your hub is in.