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.
The only way to put your Nest Protects on the new network is to change the SSID and password of the new network to match the old, or to remove them from your account then re-pair them to the new network.
- Actual quotes from What should I do when I change my Wi-Fi network password or get a new Wi-Fi router?
I was able to easily change the WiFi network and password for all of the devices in my home except for the Nest smoke alarms. Unfortunately, I have 6! To change their WiFi settings, I have to remove all devices from my Nest account using the app, physically remove 6 smoke alarms from the ceiling, and re-add all 6 devices back to my account following the initial installation procedure.