So I removed the netonix-manager package and any netonix directories I could find and then re-installed the package and it looks like it's working now. Strange.
cd /opt/netonix-manager/
sudo service netonix-manager stop
sudo service netonix-manager status
sudo apt remove netonix-manager
cd ..
ls -la
sudo rm -rf netonix-manager
sudo updatedb
sudo locate netonix
sudo rm -rf /var/lib/netonix-manager
sudo apt purge netonix-manager
sudo apt autoremove
sudo updatedb
sudo locate netonix
sudo dpkg -i netonix-manager_1.0.10_all.deb
sudo service netonix-manager status
sudo reboot now
and
https://<server_IP>:3443 now works
netstat -an | grep 3443
tcp6 0 0 :::3443 :::* LISTEN
tcp6 0 0 172.16.2.30:3443 172.16.37.10:9536 ESTABLISHED
tcp6 0 0 172.16.2.30:3443 172.16.37.10:9540 ESTABLISHED
tcp6 0 0 172.16.2.30:3443 172.16.37.10:9541 ESTABLISHED
tcp6 0 0 172.16.2.30:3443 172.16.37.10:9559 ESTABLISHED
tcp6 0 0 172.16.2.30:3443 172.16.37.10:9560 ESTABLISHED