I just replaced my cable modem, and only certain devices can get an IP address from the modem. I'm not sure why, but my router I was using no longer can get an IP from the Cable modem. I had to switch back to an older router that I had lying around just to get internet access. When I log into my account, the serial number of my cable modem is listed as the MAC and not the serial. Not sure if the 2 are related. Not sure why certain devices get IP's and other's dont. Tested multiple devices.
Are you power-cycling the modem each and every time that you change the device that is being directly connected to it ?
You should call in to customer service and get the MAC addy/ serial number thing straightened out.
Try suggestion number two.
Is IPv6 Required on my router to get an IP address from the comcast DHCP servers. I found out my router does not support getting a IPv6 Address via DHCP and I'm wondering if that might cause my issues for connectivity.