You need to first remove current dnsmasq.
opkg remove dnsmasq
It's also good to backup your current dhcp cofiguration before installing dnsmasq-full
mv /etc/config/dhcp /etc/config/dhcpOLD
Then you can install dnsmasq-full
opkg install dnsmasq-full
After you installed it you need to add this to dhcp config file under section config dnsmasq
option dnssec '1'
After you do this you can only use resovers which support DNSSEC. About the location I'm using multiple resolvers and some of them are from Australia (I live in Europe). As long as you have enabled dns cache entries, you are good to go.