If you did not use the same SSID Wifi name and password you need to do a factory reset.
There is a support page over HERE about your topic.
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.
If this doesn’t work, or you want to keep your new network name or password, you’ll need to update the Wi-Fi details on each of your Nest products.
In the Home app, Wi-Fi information for cameras and doorbells can’t be updated, so you’ll need to factory reset your device and set it up again in the app. In most cases, you’ll remove your device from the app and add it back.
Important: Make sure to save any important video clips before you start. When you remove your device from the app and factory reset it, you’ll lose your video history.
Tip: If you set up your camera in the Home app (and it isn’t compatible with the Nest app) and you want to keep your video history, don’t remove your device from the app. Factory reset your device manually and set it up in the Home app as a new device. You’ll have two versions of your device in the Home app, one with older video, and one with new video.
I recently changed my router and had to remove the device and do a factory reset - now the doorbell will not connect. I follow the instructions - and the doorbell isn't identified as a new device so I have had to add it manually. Having done this - it then gets stuck at the stage where it asks you to "plug in the doorbell indoors". I do this - it chimes, but the light never changes from white to blue.
The doorbell is less than 5 months old.
It is a massive frustration and failure. There was another thread dedicated to this issue but it has been closed. I don't want to have to send it back - but surely it has to be a better product than this.
So having fiddled around with the doorbell some more I've managed to get it to work -
The key is when you are hard resetting the device, keep the pin depressed for around 10+ secs (not 5 seconds as the instructions suggest) - the doorbell light will blink YELLOW before appearing to turn off.
At this point you should be able to add the device with the new wifi network as per the initial setup.
@Laura11 thanks for posting here in the community. I'm dropping by to ensure that everything is covered here. Were you able to read the information and check the links shared above? In case you have an update, you know where to find us.
@AdamCrozier sounds like you've been through a lot of troubleshooting steps. Anyway, we appreciate you sharing here the steps you've done to resolve your issue and we're glad to hear that it's added now.
Let us know if you have other questions in mind.
I appreciate the help, RXShorty.