My Setup

My router is the Asus RT-AC66u, but the modem is the OpenReach white box of fun.

bt-modem

Firmware

IPv6 is only supported in Asus router's from 3.0.0.3.78 - make sure you are up to date to get IPv6 and the associated security fixes in firmware updates.

Settings

After updating the settings you'll need to reboot the router (possibly by the power button, as I did, if you can't reconnect to the router straight).

Test

http://test-ipv6.com/