Pi-hole ipv6 DNS on Netgear r7000 not accepted (SOLVED)

I have a netgear r7000 router and am getting "The Primary DNS Server is not valid; it has to be a global unicast address." when I try to override the ipv6 DNS. I know this isn't pi-hole specific but wanted to see if anyone else has come across this. I found one thread discussing it on Netgear's forums but it didn't have a solution: Solved: Re: Setting IPv6 DNS - NETGEAR Communities

It looks like this router won't accept local ipv6 DNS redirects?

EDIT for anyone interested I ended up installing xwrt-vortex (Home | Xwrt-Vortex) firmware which allows local DNS hosts like pi-hole.