GL GUI → Network → DNS. Toggle both. There’s no visible indicators other than those. logread -e rebind will tell you if you get any hits… it’ll flood the log, to boot.
If you’re going as far as poking around in /etc/config/dhcp & /etc/config/network you might as well use dnscrypt-proxy2. You’d have to uninstall whatever version GL has onboard if it’s < v2.0 & the GL GUI will no longer reflect the DNS settings but you’ll know ‘under the hood’ your DNS is (eventually, once config’d) using DNSSEC, DOT/DOH, etc.
dnscrypt-proxy2’s allowed-names.txt, allowed-ips.txt &/or cloaking-rules.txt will probably give you what you need.
opgk list-installed | grep dnscrypt
Make a backup beforehand, of course.