I spent half of my day for two days just trying to reach technical support.
Some websites like American Express and Steam are blocked. Access Denied. A technician fixed the problem by changing the xFI modem and putting a noise filter. Few hours later, problem came back. Trying to reach Xfinity Support again, I got into an agent, noticed that the problem isn’t from Xfinity but it’s from their “higher” company or something which seems to imply how huge of a problem this is. He connected me to that company, after an hour later, as I was about to talk to them. Call ended. Now I have to restart the call again, with this stupid bot telling me that they should restart the gateway remotely for HUNDREDTH TIME and won’t even send me to an agent to talk to.
After 3 modem replacements, THOUSANDS OF GATEWAY RESTARTS, dozens of calls, A TON OF HOURS SPEND JUST TRYING TO GET THIS THING FIXED, one technician coming, several attempts of getting these sites work on different devices, I HAVE THE SAME PROBLEM I GOT A WEEK AGO. My expectations from Xfinity are low, but holy cow!
Try disabling the Xfi Advanced Security feature as a work-around for now and see. It's buggy ! And there is currently no way for an end-user to be able to whitelist anything;