Sb8200 not working

Hello yesterday I tried to replace an older surfboard modem that was working fine, with a new sb8200. I called in and the phone person said it was all set up and active. But I could not get internet access. I was told they could see that it was online but when plugged directly in with ethernet to my laptop there was no internet. Comcast did not have a solution and suggested going back to the old modem, which worked fine again. Any suggestions on getting the sb8200 to work?