You will need to setup the ReadyNAS for passive (PASV) FTP. To do this, go to the Services page and setup the options under FTP. In this case, you will need to select a passive port range (in most cases, a small range of 10 to 20 ports will do) and forward it from your router to the NAS. You will also need to enter your public domain name or IP address in the Masquerade field.
For more information on passive FTP, go here: http://slacksite.com/other/ftp.html