In console: nano /etc/pure-ftpd/pure-ftpd.conf.
Uncoment PassivePortRange 30000 50000
Uncoment ForcePassiveIp "your static IP address"
Restart pure-ftpd service pure-ftpd restart
P.S. Don't forget to add 30000:5000 in CSF Firewall Configuration (TCP_IN and TCP_out)
In filezilla client , set the FTP Connection to Passive (recommended)
Edit>Settings>FTP>Passive
Let me know if you solved it.