Our SB8200 Cable Modem is what Comcast/Xfinity requested we get. This modem has two ethernet connections at the back in addition to the coax cable connection. Why can't we connect a device to the second ethernet port? It does not seem to work. Is there a way to fix this problem?
You would need to have two public / WAN IP addresses assigned to your connection for that. That port is meant for link aggregation to give greater than gigabit speeds via DOCSIS if and when cable companies ever offer it.
Call 1-800-comcast and ask a rep if it is even available in your local area / system. Many areas don't even offer it. The is a additional small charge. It may be called an "IP pack".