Router behind router issue

We have a 2wire router that we use as part of our AT&T Broadband system for our tvs, dvr and wireless network. I am able to easily set up TC to "join network" and disable wireless. I am currently doing my first back up... yea!

But... I had read several earlier posts about using TC as a 802.11n only network. when i tried to do this. i would get an error saying that our existing router detected a "router behind a router," and asked me to resolve. i thought i had hooked everything up according to the previous posts.

any suggestions? also, if i do this. my other computers will not connect (husband's G4), will it? It does not have 802.11n.

thanks for any suggestions! Also, our internet connection seems very slow now.