By default, NAT is used and RFC 1918 addresses are assigned to the clients for IPv4 and RFC 4193 addresses for IPv6.
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).
First edit /etc/vpn-user-portal/config.php and set the xRangeFour and/or
xRangeSix addresses as appropriate, where x is either w or o depending
on whether you are configuring WireGuard or OpenVPN.
Next, modify the firewall according to these instructions.
To apply the configuration changes:
$ sudo vpn-maint-apply-changes