Correct Way to Set DNS Server

adblock has a tick box for that.... for users who are GUI inclined... it's "lan" zone only... but you could diff to get the rules

Force Local DNS
 
Redirect all DNS queries from 'lan' zone to the local resolver, apply to udp and tcp protocol on ports 53, 853 and 5353.
opkg update; opkg install adblock luci-app-adblock diffutils;
iptables-save > /tmp/prednsredir;
echo "go and tick option";
iptables-save > /tmp/postdnsredir;
diff /tmp/prednsredir /tmp/postdnsredir