I believe the setup process for the Chromecast device is a direct WiFi connnection from a wireless device (tablet, phone, etc.) to the Chromecast dongle to allow entering the actual WiFi password in the dongle and selecting the WiFi network's SSID ( saw this temporary Chromecast WiFi network in my WiFi network monitor), at which time the dongle switches to it and shuts it's own temporary WiFi configuration network down. Once I figured that out, I was able to set things up fine and then Chromecast connected immediately to my normal WiFi network. Note that I have the old Anctiontec router with uPNP disabled.
This worked just like setting up a wireless WiFi extender. Once the extender knows the actual WiFi network information, it shuts down it's own temporary WiFi network that is uses to allow the configuration.