1- go to "Advanced" tab
2-click "SAMBA" from left panel and check "Enable SAMBA" and "Enable USB Storage" with other option.
3-click "FTPD setting" and check "Enable FTP Server" and "Enable FtpServer for WAN" with port 2121.
4-click "FTPD Account" and create an ftp account.
5-click "DMZ" and check "Enable DMZ" and fill "DMZ Host IP Address " with Static ip.
6-click "Port Forwarding" and add port forwarding "external port" with value 21 to "internal port" with value 2121 with static ip for "Server IP Address".
now you can go to ftp address from browser and login with created ftp account to usb storage.