@beri,

If you are having trouble connecting devices to your network after they were connected to another (especially one with the same SSID), the first step is to always factory reset your devices as this will clean any network information that was stored on the device. I do not know how to reset these devices, but it should be either a combination of light switch presses, mute presses, or it will be a pin hole button that must be held. 

I have known Google Devices to be finicky when it comes to reconnecting to networks, even if it was already connected to the network. This is the only reason I actually know that these devices can be reset. If I recall correctly, Google Home Minis require you to mute and unmute the speaker, and then the device will tell you it is resetting.

  @beri,

Sorry to hear that it still isn't working as intended.

I am really surprised that factory resetting the devices does not allow you to connect the devices to a new network. The only reason that I can think of, is that Google is saving the network information in the cloud and then using that information for the new SSID.

Is the new network on the TP-Link router using the same SSID as the previous network? If so, google may have saved the network information and is trying to connect using old credentials - This may be the reason why you can set up the devices using the guest network.

Also For Clarification, have you tried to set up the devices while the Nest Router is Disconnected from the network? Having two routers operating on the network simultaneously can cause issues such as Double NAT and may prevent some devices from connecting to the network. 

As I am not familiar with the Google Nest Wi-Fi Interface, is it possible that these devices are being bound to the router and that they need to be unlinked? I do not know if Googe binds the devices to the network to try and create a unified home.

  @beri,

In this case, can you ensure that the devices you are attempting to connect are compatible with the selected security type? Most Devices require WPA2/WPA3 security nowadays, and the lack of these methods may prevent the devices from establishing the initial connection to the network.