We can add DNS settings using option 6, on the LAN >> DHCP >> ADV Settings Define additional DHCP options, for example "6,192.168.2.1,192.168.2.2" which advertises different DNS servers to clients.
There is yet another place and that is LAN >> ADV Settings use custom DNS servers
OK. So, it appears that the DHCP option 6 is a reliable option.
I read some more and found out that making use of BanIP to use the DoH and DoT public list and force the use of port 53 thereby making it a leak proof solution... Is that correct?