DirectAdmin can run on a LAN/NAT, but just requires some non-standard steps to get it running.
It requires the of loading the external IP into the internal/LAN device. Operating should still be fine via the local IPs.
echo 1 > /root/.lanto have DA download the update.tar.gz and license.key more easily (skips the --bind-address option).
lan_ip=192.168.1.2where you'd replace 192.168.1.2 with the LAN IP value that should be used for outbound connections.