Nighthwak CM1150v No Internet Direct to Computer

Hi there,

           I have a nighthawk cm1150v and a nighthwak ac1900 wifi+router. I have no issue to gain internet connection wireless or wired from router to my computer. If I plugged in my computer direct to the modem by using same port connect to router (port 1), my computer cannot get internet access. I tried both mac and windows. Does anyone know why? Despite of security issue, i think my computer should have the internet if connect the same port on modem. I checked my comcast account which didn't show my wifi+router info cause I'm afraid ISP store the info and doesn't issue internet service to any other device with different mac address. Thank you.