In case you want to use routable IPv4 and/or IPv6 addresses you need to modify the configuration and make sure your upstream router routes the appropriate range(s) to your VPN server's IP address(es).
/etc/vpn-server-api/config.php and set the
addresses as appropriate.
Next, modify the firewall according to these instructions.
To apply the configuration changes:
$ sudo vpn-maint-apply-changes
If the command is not available, install the
vpn-maint-scripts package first.