DHCP Reservations (Fixed IP) in UniFi Controller 5.6.x

After setting up a UniFi Security Gateway last week, I noticed the latest stable UniFi Controller build was up to v5.6.22, while my server was running 5.5.20. Apparently 5.6.x has been around for months and the controller does not provide update notifications due to backwards incompatibility, as the UniFi development...