Você precisa definir o IP externo do seu servidor para que o modo passivo funcione.
No IIS: (passo 5 para o modo passivo, outro para o modo ativo)
How to configure the global firewall settings for the FTP service
1.Open Internet Information Services (IIS) Manager:
•If you are using Windows Server 2012 or Windows Server 2012 R2:
•On the taskbar, click Server Manager, click Tools, and then click Internet Information Services (IIS) Manager.
2.In the Connections pane, click the server name.
3.In the Home pane, double-click the FTP Firewall Support feature.
4.In the Data Channel Port Range box, use one of the following options to enter a port range for passive data channel connections:
•Use [low port]-[high port] for a port range, where each port must be between 1025 and 65535. For example, 5000-6000.
•Use "0-0" to configure the FTP server to use the Windows TCP/IP ephemeral port range.
5.In the External IP Address of Firewall box, type the IPv4 address of the Internet-facing network adapter of your firewall.
6.In the Actions pane, click Apply.
Se não estiver lá, você precisa ativar a Extensão de FTP.
Windows Server 2012 or Windows Server 2012 R2
1.On the taskbar, click Server Manager.
2.In Server Manager, click the Manage menu, and then click Add Roles and Features.
3.In the Add Roles and Features wizard, click Next. Select the installation type and click Next. Select the destination server and click Next.
4.On the Server Roles page, expand Web Server (IIS), and then select FTP Server.
Note: To support ASP.Membership authentication or IIS Manager authentication for the FTP service, you will need to select FTP Extensibility, in addition to FTP Service.
5.Click Next, and then on the Select features page, click Next again.
6.On the Confirm installation selections page, click Install.
7.On the Results page, click Close.