WARNING: I don't recommend doing this, to validate DNSSEC locally you must run a local DNS forwarding service. You can keep using systemd-resolved or any alternative, but don't point resolv.conf directly to a remote DNS server. By doing this you're not validating DNSSEC, which is a security issue.
https://www.sourceware.org/glibc/wiki/DNSSEC
systemctl disable systemd-resolved.service
systemctl stop systemd-resolved.service
# check if resolv.conf is pointing to resolvconf
ls -la /etc/resolv.conf
# lrwxrwxrwx 1 root root 27 May 7 16:15 /etc/resolv.conf -> /run/resolvconf/resolv.conf
# if not, delete /etc/resolv.conf and symlink it like this:
rm /etc/resolv.conf
ln -s /run/resolvconf/resolv.conf /etc/resolv.conf
# this will remove the resolved stub resolver entry from resolv.conf
resolvconf -d systemd-resolved
# fix dhclient scripts
chmod +x /etc/dhcp/dhclient-enter-hooks.d/resolvconf
# ifdown/ifup your interface to regenerate resolv.conf (or systemctl restart ifup@eth0)
ifdown eth0; ifup eth0
# check /etc/resolv.conf has the right settings
my post from https://askubuntu.com/a/1336755 (slightly modified to remove unnecessary steps)