How can I fix an "Unable to extend wireless network" error? I get this every time I try to extend my home network with my Airport Extreme. Even when the two are sitting next to each other.

Comcast recently upgraded our Xfinity home internet service with their own wi-fi router, an Arris TG862. The signal isn't as strong as the Extreme, so I wanted to extend the network wirelessly. Every time I try, however, I get an error reading: "this Airport base station was unable to extend the wireless network." The paswords match; the two devices are in range of each other (right next to each other on the desk, for setup.) The wireless connection between the two is displayed as "Excellent". But I can't get beyond an amber light, and I can't get the Extreme to pick up the wireless signal and throw it towards the dark patches.

Any help would be greatly appreciated. Thank you, apple community!