Ok, in that case your CGN2 is cooked. Tape a note to it explaining that using the USB setup stick has resulted in an additional noname wireless network being transmitted, on top of the network it is supposed to transmit and then take it back and exchange it. That is the same thing that happened with my CGN3. The one thing that I didn't ask tech support at the time was whether or not they had the ability to really nuke the modem, and clear out all of the non-volitile ram (nvram). You could always ask, but, as I already indicated they will not understand what you are doing or why you are doing it.
When you bring home the next CGN2, set it up without using the USB stick. As I said before, throw it out. Run inSSIDer just to make sure that only your intended network is running and that you are not ending up with a recycled modem that has a pre-existing problem. This time you should, or will end up with all of the wifi power transmitting the intended wireless network instead of being spit three ways. That should make a difference in your wireless reception around the house. Will it be enough to reach everywhere? Only you can judge that by using inSSIDer to look at your network and those of your neighbours, and determine if your network has enough receive power to work everywhere in the house. What you see at that point might lead you to consider buying a third party router. If you need a guest network, I don't think you will have any choice as you have already seen what will happen by using the USB Setup stick.
The remaining item of course is to have a tech come out and check out the external cable and connectors.