You can manually edit the database to set a different IP or interface. But it has to be a valid IP or interface that actually exists on the server itself. If your system only has an internal IP configured, which is NATted from outside on a public IP to the private IP, then the issue must be resolved there.
In other words, if the public IP is not on your interfaces, then you can't configure that.
Use this command to find out what your current settings are;
/usr/local/openvpn_as/scripts/confdba -s
And these to change it to listen to all addresses;
/usr/local/openvpn_as/scripts/confdba -mk admin_ui.https.ip_address -v all
/usr/local/openvpn_as/scripts/confdba -mk admin_ui.https.port -v 943
/usr/local/openvpn_as/scripts/confdba -mk cs.https.ip_address -v all
/usr/local/openvpn_as/scripts/confdba -mk cs.https.port -v 943
You can replace 'all' with an interface name or IP address.
The web based interface should then become available on port 943 (
https://yourserver.com:943/)