The Comcast, Xfinity, Time Warner walled garden / gstatic 204 / captive garden / endless loop / signup redirect / certificate error and anything else you want to call this mess is a real pain. This scenario applies to anyone that owns their own equipment. This would typically be a cable modem and separate router. For example, you may have a Motorola cable modem and a TP link wireless router. Basically any combination of new or upgraded gear can trigger this problem.
Whenever you upgrade a cable modem or router you may run into this issue. The first step is make sure your cable modem has been properly provisioned by your service provider. This may take some effort to get the proper help to do this. Most service provides offer a basic phone tree and bot support over text. You may need an actual human to help you with this.
Once you have confirmed your cable modem and MAC address is properly provisioned by your provider it's time to look at your router settings. Most routers ship with similar default settings which may or may not work depending on your setup.
After two different upgrades with two sets of gear we found a simple fix for the gstatic 204 issues. In both cases switching the DNS settings from “auto” to 22.214.171.124 and 126.96.36.199 solved the problem. Prior to updating these settings on the router the walled garden and Xfinity activation page would pop up on all devices from PCs to smart phones. You can usually find these settings under the network tab on your router. Every menu structure is different but your generally looking for "advanced" settings in the network section.
So give it a try. It might save you from tech support hell. Comcast wanted to send a technician out to look at the problem. It's not a connection or wiring issues. The Comcast bot support seemed to think there was an interference issue based on their diagnoses of the problem. Yeah, classic. Anyway we hope this helps anyone having a problem with this issue.