I personally have IPV6 support disabled. One less thing to care about and set-up.
I do believe that the issue is the router's firmware that even if it allows you to put internal (private) IP as the DNS, it will not route and allow that.
Unfortunately this seem that (and only due to) the firmware is the culprit here.
As I see it, there are 2 options here.
Either go the cascading route where you add another router AFTER the Netgear (and use that as your network node) or replace it for a standalone Modem and a Router ...
I don't know if there is a DDWRT or OpenWRT out there for your model, but that might be worth looking into as THAT firmware would give you the much needed freedom to make it work.