Is there a way to stop the router from assigning itself as IPv6 DNS?

[Tekneek]

I have an RT-AC3200 running 380.66_4 and regardless of what I put into the DNS fields on the IPv6 page, all of the devices pick up the router's IPv6 address as DNS. I run my own custom DNS on the network and prefer that all primary lookups go through it, with the router being secondary (effectively a fail-over for the network - it will then look externally).

Is there a way to force the router to push the IPv6 custom DNS entries out via DHCP?

This works easily with IPv4, as the entry provided in LAN -> DHCP Server -> DNS Server 1 pushes out to all of the devices as you would expect.