If using Passive FTP on your cpanel server, and also using the platform firewall to help secure your server, you will need to specify the port range for Passive connections on both the server and firewall manually.
Follow this link for how to configure the range in FTPd: https://documentation.cpanel.net/display/CKB/How+to+Enable+FTP+Passive+Mode
Once you have chosen a range, in the dediserve GUI, click on Network / Firewall and add a rule to
0.0.0.0/0 addresses (assuming you want public access) and then the port range in the format from:to to match your FTP config above.
Click to apply firewall changes and all should now be working correctly.