Further tips:  Get a faster router; get a new ISP as suggested (AT&T's equipment really lags behind on technology), and then replace your desktop PC with built-in N adapter.  AT&T didn't tell you about the newer N-class router, as the training of their Customer support reps has severely deteriorated over the years.  That's a 2-ended failure of AT&T; lag of technology premises equipment, and lack of internal training.  Very few people stay with them longer than 1 year these days; at least in my part of the country-West Coast. 
As far as router brands go--there are only 3 brands: Cisco, Cisco/Linksys, Netgear to buy.  Anything else will produce many kinds of network problems I won't go into here.  Needless to say all of those...