Here: http://www.snbforums.com/threads/tp-link-archer-c7-good-for-ipv6.22417/
And I asked them too, here was their reply:
Thank you for choosing TP-LINK, This is Cara from TP-LINK Technical Support.
I am sorry to let you know that the Archer C7 does not support the SPI Firewall over the IPv6...