@Andiamo
Well, you have two different issues. One is that your cameras went offline. The second is that you changed your Wi-Fi network. On the second issue--changing Wi-Fi settings--Google Nest does not make it easy. Their first recommendation is this: "The simplest fix for most Nest products is to update your home network so that the network name (SSID) and password are the same as before. This will usually work even if you’ve purchased a new Wi-Fi router as long as the manufacturer settings are the same." (https://support.google.com/googlenest/answer/9223711?hl=en#zippy=%2Ctry-the-same-network-name-and-pa...)
Are you able to change the network name (SSID) and password on your new router/modem to the same values you used on your old router/modem? That should get you past the "remove camera from previous account" error. Otherwise, Google Nest's next recommendation (see link above) is to first remove the camera, but you'd need to be on your old Wi-Fi network to do so, and then do a factory reset.
I don't know about the offline problem. I've seen similar reports from other customers, but having 3 faulty cameras seems a little improbable (but not impossible). If you can get past the Wi-Fi issue, can you temporarily locate your cameras close to your router or access point, to at least find out whether the problem is your Wi-Fi signal strength? (Or maybe your new router will solve any Wi-Fi signal strength problems.)
