I own a E4200v2 and wanted to do this some time ago. But then I realized again how "unsafe" FTP actually is. First off the E4200v2 does not even support sFTP.
then for passive FTP one has to open a lot of ports:
FTP server's port 21 from anywhere (Client initiates connection)
FTP server's port 21 to ports > 1023 (Server responds to client's control port)
FTP server's ports > 1023 from anywhere (Client initiates data connection to random port specified by server)
FTP server's ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client's data port)
(»
slacksite.com/other/ftp. ··· #passive)
Not my cup of tea. Not worth it IMHO.
I got myself a proper NAS server and connect with VPN when I need access to my files from outside my network.
TA