...or any other alternative...
[/quote]
On my test server the default firewall configuration allows access to
https://serverip/ from a captive portal connection, so I suggest double-checking that you cannot reach the administrative GUI through the captive portal connection.
It sounds like you're running the server headless. I suggest that if possible, you use (or have the option of using) the local Zentyal desktop during configuration at least. Once the server is running as you want, then you can go back to a headless server.