I have three Deco x20 units working great in AP mode in my house. I'm expirimenting with IP/Wifi camera's and want to build a completely separate network that these devices can use. In the Deco app, I removed 1/3 of the units and left the original network operational with the other two units. Now I'm attempting to setup the other unit as a separate wifi network but I can't seem to prevent the unit from getting automatically joined to the old network. I gave the single unit a wan port on my firewall appliance that is firewalled off from the other networks but everytime I get to the creating ssid stage the light blinks red then I get an notification that a new deco joined the original network. I've reset the device multiple times, waiting for the blue light and gone through the setup but every time it just gets added to the old network.

How can I prevent this Deco from joing the old network after it's been removed? And is there anyway to set one of these up without an active internet connection?