Within luci there is a firewall rule to forward port 53 (DNS) from the router to the port of AdGuard - I guess you need the same rule for Adblock-Fast.
Unfortunately, I don’t have access to the interface right now, so lacking a screenshot. But I am sure you will find it.
Doesn’t stay in memory – creates the list of blocked domains and then uses Dnsmasq/SmartDNS/Unbound and firewall rules to serve NXDOMAIN or 127.0.0.1 reply or to reject access (depending on settings) for blocked domains.