So in a previous release of Neon prior to 5.15 I managed to get my hostname registered on DNS by adding these two lines to /etc/dhcp/dhclient.conf

send fqdn.fqdn "hostname.local";
send fqdn.server-update on;

However this doesn't seem to work on a fresh install of Neon. Has something changed? I read that Ubuntu 1804 switched to netplan, is Neon on this too?

Yes, Neon is 18.04, so it is using netplan.