If you have hw rev A do not bother with ipv6, will not work; ULA trick works some times, but after a router restart you have to do it again. I have tried 3 units, all are the same, so no point in changing it, except maybe to a rev B (not sure if that has proper ipv6, but here is hoping)
Maybe a manual routing configuration is a more permanent solution, but I don't know exactly how to do it. For me is pointless anyway, as they resolved pppoe offload issue just for ipv4, on v6 is maxing out aroun 250 mbps.