I have configured IPv6 over cox connections at over 50 sites, it works fine for docsis service over coax, it works fine for business fiber service, it however does not work and has never worked for gigablast fiber for the home at any site I have tried. I have tried IPv6 DHCP requesting /56 /60 /64 prefixes, I have tried using SLAAC to get a subnet, static IP's are not an option with residential service, so at a loss. Many others seem to be facing the same issue.
Maybe it's not available in your area, but it seems it's available for fiber in general. I saw your post on Reddit and 2 people from AZ said theirs was working. Are you in AZ? If you want to ask Cox directly, email them via the welcome thread.
Are you using a retail router or something more complex like pfSense or ddWRT? If you want to rule your side out you could try renting their gateway for a bit and see if it works. Their gateway has no IPv6 settings to configure so if it's available in your area, it should work on the gateway plug and play.
