What is wrong with my wifi network?

This has me and Comcast stumped. I have the most-basic home wireless network you can get: Cable>cable modem>Airport Extreme. Worked fine for a year but, about a week ago, the wifi stopped working. I did the basic diagnotics, resetting the Airport Extreme, resetting the modem, etc. Nothing. Called Comcast. They said the modem was fine (they can test it remotely and even sent two guys out too check it). Well, since then, I have tried everything I can, replacing every step in the system with different items—two different modems, two sets of cat5 cables, three different wifi routers: Airport Extreme, AP Express and a Linksys N something or other with a Mac installer, and two different computers. The result is always the same (and here’s where it gets confusing):

I can get online with no trouble when directly connecting a computer to the modem with an ethernet cable. BUT, when I try to connect a wifi router to that same cable, it always says that it cannot detect any connection to the internet. Any ideas? I can’t imagine that I have three routers that are all defective at the same time.