@JMLM
You could try contacting Support, but my guess is that they cannot help you install your doorbell without either the QR code or the 6-character alphanumeric setup code. (That's why we got in the habit of writing down all of our setup codes.)
To contact Support:
1. Go to https://support.google.com/googlenest/gethelp.
2. Select a category/product family, and type in a brief description of the issue, then click "Next".
3. Click "Other", then click "Next step".
4. Under "Resources", just click "Next step". (Clicking one of the listed links will bring up a Help page page.)
5. Under "Contact options", choose to get a phone call or chat with the team, depending on availability in your country.
