Why can't I connect to Xbox Live?

My 360 won't connect to live. I usually connect to Live by connecting the Xbox to my laptop and bridge the wireless and wired connections. Our Internet connection recently started going ridiculously slow, so we got a new cable modem and a new wireless router. This fixed our internet problems, but now I am unable to connect to live. The error message tells me I need to set the MTU to 1364 or higher. When I went to set it, it was 1500. I changed it to auto and got the same result. If what I just said makes any sense at all, could you tell me how to fix this?