Comcast does have IPv6 on their entire residential network, they've been touting that for a year or two.

There are several ways they have to check and test, (quick way, more, see RH column here). Sounds like in your case you want to break it down and see if the problem is your modem or router or the hosts.

To check IPv6 to the modem, disconnect the network cable between modem & router, reboot the modem (that means power it down and power back up). When the modem has rebooted and is online, connect the network cable directly from the modem to your PC. You should get an IPv6 address within about 30 seconds of connecting the network cable.

That is a purpose built test Comcast uses, their servers detect a single host and give it one IPv6 address, a so-called /128. I forget the actual details, but you may get more than one address, a link local/fe80...:, and one permanent and one temporary global/unicast, 2601.... If your host gets this, then obviously IPv6 works to the cable modem.

For a home gateway/router, the correct settings are:
WAN side:
- Enable DHCPv6 on the WAN.
On the LAN side:
- Enable SLAAC, and PD (prefix delegation enabled). Sometimes the PD settings are on the wan side. Use a prefix size of 64.
- You also need DHCP enabled on the WAN side (although it's not used the same way as IPv4), and set the "other flag", and disable the "managed flag." This is how some values get from the router to the hosts, such as IPv6 DHCP servers, IPv6 default gateway, etc.
- Enable RADVD, the commonly used acronym for Router Advertisement Daemon.
- Don't enter any advertisement prefixes, the router will get these from the WAN side.

Reply back if you get stuck.