I own an Arris SB8200 Modem have a 1000 mbps internet plan and I am only recieving an IPv4 IP address. I have called comcast technical support twice and they both advised my modem is provisioned correctly and show that I should have an IPv6 address; however when I test it online it clearly shows no IPv6 address. I contacted arris who advised me there there is not a setting on my modem to enable this and comcast should be provinding me with native IPv6. I got a suggestion from the comcast agent to try and type the question in this forum for answers?
O/k so the next step is do you get IPv6 connectivity with a direct connection to the 8200 cable modem ? You'll need to power cycle the modem.
Check the settings in the router. It may be disabled or something is set incorrectly. Check your user manual.