Hi,
For those who encounter such issues, you need to open FTP port range for passive connections from the CSF config file. Find your FTP config file, for example pure-ftpd will be /etc/pure-ftpd/pure-ftpd.conf, and search for PassivePortRange. Set it to something like this:
PassivePortRange 30000 30050
Restart the ftp server. Then go to /etc/csf/csf.conf and find 'Allow incoming TCP ports'. Add 30000:30050 at the end of the TCP_IN line, save the changes and restart csf. Now the connection should work.