No ip address when computer is plugged into modem

I recently upgraded to gigabit service and I have not been able to get more than 250mbps on any device in my house. A technician came today and said the speed is good at the modem and suggested it may be my router. I am using a netgear nighthawk cm1200 modem. The problem I am having is that I can't test this for myself. My router will pull an ip address from the modem, but I do not get an address when I plug my computer directly into the modem. I don't really have a problem buying a new router if that really is the problem, but I would like to verify that before spending anymore money. Can anyone suggest what to do in this situation?