>> I presume this is probably firmware specific. Have you tested with various
>> firmwares to confirm the problem?

I suspect not. This problem would explain some very bizarre DNS problems
I've experienced at home when using IPv6, with another DSL box from the
same vendor, not used with Xtra.

I never diagnosed it, but I bypassed it by disabling use of the
resolver in the CPE and going straight to the ISP's resolver.

I believe they are all using dnsmasq as the resolver, and old 2.4 linux kernels for network stack. It would not surprise me if the root lies in one or the other.

In either case I am fairly certain a customised firmware would solve the problem. Alternatively, just disable dns relay in dnsmasq on the router and hardcore an upstream one as you suggested.

Kind regards

-JoelW