Xfinity does not support port forwarding

I have Xfinity home internet and had set it up with my own Arris SB8200 modem. I was experiencing frequent connection drops. The tech who came to my house told me that an xFI Gateway would resolve my problems. I did that. Now I'm trying to use the port forwarding feature from the gateway UI, but whenever I enter the target system, port, and protocol, the configuration fails. I contacted level 1 support, and after 45 minutes the confirmed that the issue existed. They transferred me to "advanced support", who spent another 30 minutes diagnosing and confirming that port forwarding was not working. Today, another tech came to my house and told me that port forwarding wasn't a supported feature. I showed him the Xfinity user interface menu for port forwarding (https://internet.xfinity.com/network/advanced-settings/portforwarding).

The Comcast tech told me that many customers had complained about this, but that there was nothing he could do. I asked who could help, and he called is supervisor, Eric, who told me that Xfinity only provides the network connection and does not support configuration of devices. I pointed out that it is an Xfinity device and asked who does support it. He said it wasn't his problem, and that I should buy my own modem if I wanted port forwarding. I told him that I only had the xFi gateway because a Comcast tech told me it was needed to be fully supported. He didn't care and made that very clear.