FTP passing local address to WAN client?

My FTP Server Configuration is set to LAN and WAN. I am using VSFTPD and when I connect using WinSCP it shows WAN side connection is encrypted with TLS 1.2. However; during upload or download WinSCP looses connection. The reason why it’s loosing connection is: "The address that is being passed to the WAN side client is the server’s local IP address instead of the WAN IP address". How can I tell the router to pass the WAN IP address instead???

1. I am using FreshTomato Firmware...

FTP passing local address to WAN client?