If you want to tell ROS about IPv6 DNS servers then this is done as follows:
/ip dns
set allow-remote-requests=yes cache-max-ttl=1w cache-size=2048KiB \
max-udp-packet-size=512 servers=\
8.8.4.4,8.8.8.8,2001:4860:4860::8888,2001:4860:4860::8844
(using Google’s servers, for example). If this isn’t what you wanted, then what do you want?
Which RouterOS version are you using? Support for supplying DNS servers via DHCPv6 is supposed to be in the RouterOS 6 beta but I don’t think it has appeared in any RouterOS 5 versions.