In IPv4 mode, you can set custom DNS servers with "Automatic Configuration" for DHCP.
In IPv6, you cannot do this. There is no option to set the DNS servers. Also if you try to set "Static DNS", under "IPv6 LAN Configuration", the changes are not reflected under "System Summary". They are set instead to my ISP's DNS no matter what I change them to.
Also I am on the latest firmware 1.0.3.22.