Need help with Xbox live connection issues...?

Ok so to start off, my live connection is normally a little laggy. I use a belkin f5d8236-4 v2 router and scientific atlanta 2100 modem. When my Live connection isn't working, I usually just reset them both. For some reason now, its not working. Yes, I have opened port 3074 from my router and opened the nat. The belkin router doesn't allow me to change the mtu settings, so no help there. What I find odd is that the Send and Receive lights on the modem aren't on, yet I still have an internet connection, just not an xbox live connection. Also, my wifi is working because I can connect using my laptop. Do you think this is a cox issue or what else could it be?