Hello,
It depends on your VPS service provider. Which one are you using? You should make sure that you can connect to port 4000 on your cloud server (maybe there are some settings in your VPS that forbid this) if you have NoMachine Free. Or you may use SSH protocol with port 22 if you have one of the Enterprise family products. As for Direct Admin and firewall – it’s possible, but to make sure please give us the output of command (ran as root):
iptables -L INPUT -n –line-numbers