Google Nest's first suggestion is to use the same network name (SSID) and password on your new modem/router that you used on your old modem/router.
Otherwise, you should remove the Google Nest device from the Google Nest app or Google Home app while you are still on your old Wi-Fi network, do a factory reset of the device, and then re-install it.
meemee0427, I’m sorry to hear you’re having trouble with this. No worries, we'll check this out. When did the issue start? Are you getting any error messages? How far is the camera from the router? Are there other devices connected to the same Wi-Fi where the camera is connected to? Also, are there devices or appliances (such as baby monitors, microwaves, cordless phones, etc.) near the camera?
Check out this link to Troubleshoot offline Nest camera or doorbell.
I appreciate the help, MplsCustomer.