Hey imagecanon,
Sounds like you're having issues with your Wemo Doorbell not responding or that it's no longer showing in the Home app.
Since this is a Belkin product, they'll be best equipped to assist you with any issues you're facing with it and they have a great resource below to help out: Troubleshooting issues on the Wemo Doorbell
We've also got some steps from Apple to help out with adding it back to your Home app here:
"Add accessories with your iPhone or iPad
To add an accessory to the Home app, use your iPhone or iPad. You can't add accessories to the Home app on Mac.
- Open the Home app and tap Add
. - Tap Add Accessory
. - Use the camera on your iPhone or iPad to scan the eight-digit HomeKit code, or QR code on the accessory or accessory documentation. If you have an iPhone 7 or later and see
on your accessory, hold your iPhone near the accessory to add it. - When your accessory appears, tap it. If asked to Add Accessory to Network, tap Allow.
- Assign your accessory to a room and name it to help you identify it in the Home app and control it with Siri.
- Tap Continue, then tap Done."
All this and more is available here: Add an accessory to the Home app
We hope this helps out.
Take care!