DirectAdmin runs on port 2222 by default, for restrictive firewalls this port will likely be blocked hence you’ll be unable to login to the control panel. You could of course use a VPN or SSH tunnel, but unless your running those types of services on standard ports like 443, the most restrictive firewalls aren’t going to allow such actions. Instead what you can do is use a reverse proxy and run DirectAdmin on port 80/443 with a bit of a custom Apache configuration.