The Problem with Android in regards to DNS: you just can't set your own IPv6 DNS Server on that platform, it gets changed anytime anything happens to your wifi. There is no app, even for rooted android, which can disable the operating system from changing it.

When you are stuck with a router that always hands out IPv6 Adresses and doesn't let you turn that off you are just screwed.

I don't even know if you could install a firewall appliance behind that router and strip out the IPv6 DNS Servers it advertises.