I am trying to set the custom DNS servers in IPv6 just like it is allowed in IPv4.
This is not allowed in the "IPv6 WAN Configuration" when "Automatic Configuration" is selected. However, this is allowed in IPv4, you can select your own custom DNS servers along with"Automatic Configuration" for DHCP.
Under "IPv6 LAN Configuration", when trying to change "Static DNS 1" and "Static DNS 2", the changes are not reflected in "System Summary". It is always set to my ISP's DNS IPv6 servers.
Edit: I am also on the latest firmware 22.214.171.124