Help with internet connection?

I have comcast high-speed set up, along with wireless using a linksys wireless n dual band router. The comcast high-speed as you may know runs through a modem. The modem is wired in my downstairs basement, up to the modem at the computer. This is where my problem comes in. I have an addition on my house and the wireless doesnt quite reach up there (walls, doors, stairs etc.) so i have been trying to think of a way to get good internet up there. I have tried adding a wireless repeater (worked like crap). So i am out of ideas. An access point would require plugging into a data drop which i do not have up here. Im not sure if there is an easy way to do this without alot of wiring. Please let me know of any ideas you may have. Can I maybe add another modem? I do have an rj-11 plug up here so that is possible if it would work. Thanks!