directly connecting WIndows PC to Xfinity gateway

I just had my Xfinity internet installed this morning. The technicians who did my installation had me test on a WIFI connected laptop before they left to confirm that the service was up and running. This was successful, and WIFI connections have been working great all day. They also told me that I could plug a PC with a wired network adapter directly into the Xfinity gateway device. I did not test this feature until well after they left. I cannot get this to work, even after spending quite a bit of time troubleshooting, and doing several reboots. I have a Dell desktop PC running Windows 7 Pro in the same room with the Xfinity gateway that does not have WIFI. I previously had this PC connected (more or less) directly to the modem of the Verizon DSL network that Xfinity replaced. The "more or less" part is that on the Verizon setup I had a Buffalo router wired directly to the Verizon gateway, and this PC plugged directly into the Buffalo device (which also handled my WIFI connections). I moved the network cable connected to the desktop PC from the Buffalo router to one of the ports on the back of the Xfinity gateway. It appears to have connected, it even shows up as a device in my Xfinity app. However, it cannot connect to any hosts on the internet. If I run Windows diagnistics on the internet connection it complains about lack of a DNS server. There are IP addresses coded into the configuration for the Realtek network adapter in the desktop, including a primary and secondary DNS server address. However, DHCP service is also turned on for the adapter, so those addresses should change if a connected gateway or modem device that requests it. So, at this point I do not know if there is some configuration that must be done to the Xfinity gateway to accommodate a directly connected PC, whether I might have the cable plugged into an incorrect port on the gateway, or whether I need to somehow override the IP addresses being used by the network adapter in my desktop computer to those expected by the Xfinity network (or some combination, or none, of the above). Unfortunately, while there is quite a lot of information and support documents that cover connecting to the Xfinity gateway and network using WIFI (which worked without any effort on my part) I have found virtually nothing on how to use and troubleshoot a directly connected PC. I tried using the automated assistant on the Xfinity web site with no success. So I very much need help to get this done. Where do I begin?