I’m using an ASUS RT-N66U and my ISP is Aquiss (an Entanet reseller). I wanted to get IPv6 working.
After a little bit of fiddling, the following settings seem to have done the trick:
Connection type: Native
Interface: PPP
DHCP-PD: Enable
Auto Configuration Setting: Stateless
Connect to DNS Server automatically: Enable
Enable Router Advertisement: Enable
Hope this helps anybody else in the same situation.