I am not using DD-WRT in my router. I have a TP-link C8. I had to play with different settings before I got it to work. The settings I used for Comcast are:
Get IPv6 prefix
Get IPv6 DNS server automaticlly
IPv6 Address assign type is SLAAC
Seems like I had to reboot my router before the changes took affect.
I use https://test-ipv6.com/ to test the connection.
Lets us know if this helps.
Try this: "DHCPv6 with Prefix Delegation" It's what they use.
And make the MTU "1500".