Im not sure I understand. Are you just going down the list of solutions on this page? If so, then you want to enable IGMP, not disable it. And by default it is enabled. Cox probably just removed the ability to disable the feature since I cant think of any reason why you would need to disable it. Page 78 of manual
"IGMP forwarding (proxying) is a system that improves
multicasting for LAN-side clients. If the clients support this
option, keep the default, Enable. Otherwise, select Disable.
So the real question is what kind of problem are you having connecting your Chromecast? Is it not connecting to the network at all? Slow? Intermittent? Etc