I did get this working in version 12 by adding the following in a file under /etc/powerdns/pdns.d/axfr.conf
on your primary DNS server.
allow-axfr-ips=127.0.0.0/8,<Insert secondary DNS IPs here>
also-notify=<Insert secondary DNS IPs here>
primary=yes
default-soa-edit=INCEPTION-INCREMENT
Then just run sudo systemctl restart pdns
. This, of course, is not officially supported.