Connecting My Xbox 360

Hello, I am new to this website and didn't kno where to put this topic so I will just let the admins move it if need be. I need help, I recently bought a cable modem off of ebay. It is the same type as a modem I've had for years. I've taken the old modem to my room before to play xbox live and it has connected just fine, but this new modem won't work in my room. I go to the connection menu and run the tests on the xbox and it says i have a MTU failure, and I have no idea what it means. I don't kno why a modem that is exactly like the modem I already have is having problems, but I've searched the internet throughly to no avail about my problem and was hoping someone here could help.I've read that maybe my service provider wont let me use 2 modems at once, or that the modem might be setup for a mac, also could it just be that the modem is messed up? I plug it in and it boots and everything, the buttons flash just like my other one, its an RCA DCM425 cable modem. I just want to use it connected directly from my cable cord to my xbox in my room. I've also tried to check the MTU setting through the xbox and it won't let me, I got to the first status error codes menu and then it says to click A and I do and it just takes me back to the menu to run tests. I just really hope someone can help, I just wanna play xbox!!! Lol, thanks in advance and sry about the post being so long.

jane iredale