We have DSL...hooked into our main desktop computer (it does not have
wifi)...It is A T & T .

We bought a router and it is hooked into modem for the DSL...and sends a
signal to the upstairs laptop with wifi. It is a little box with an antenna.

So both can go over the AT & T connection. The router comes with a software package to install on the laptop so it will recognize the signal. Now for us, this router interfered with IE 7.0. I don't know if all routers are the same, but for those with IE 7.0 you may have to check after loading it for error messages.
Our router installed a .dll file into its directory, and then when we upgraded IE that also installed the same file into Windows. So we had to rename the one in the router file. (my son figured this out for us). Our system crashed after we upgraded IE..it was one of those automatic upgrades.