@Mitch357
Judging from other posts in this forum, I believe the battery doorbell has a small magnet that detects whether it is mounted to the mounting bracket, and will not function properly if it is not mounted properly.
I suppose you could try contacting Support to see if they would consider providing a replacement mounting bracket under the 1-year warranty if your doorbell is less than a year old. (Given the circumstances, they might not, since the eBay seller should have included the essential mounting bracket.)
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.
