|Actually what do you mean you bought a modem? Did you call them and have them provision for it?|
Comcast provides you with one. I had one go out and had to take it to center to be exchanged. You can access it at 192.168.1.100 [at least for mine] to view errors.
There is nothing you can do to fix your modem or their line but you may need to give them facts like the errors seen in the modem
BTW its Mbps or bps [bits per second]