With NAT-Device you mean your server? If yes, then the answer is: You can't (simply) acces your server through the vpn (see softether manual, part 3.6.11)
You need a secondary interface (maybe a virtual adapter is enough, maybe not).
Or you can try to install SoftEther Client on the server and let them connect to localhost. (and use the IP of the VPN-Client instead of the vpn-server-ip)
Or use a additional ssh-connection ;)