One of the most common causes of FTP connection problems in servers using NAT (Network Address Translation) is having the passive port range blocked by a firewall.
By default, the passive port range is 49152-65534. However, this range can be changed. It is best to check your FTP configuration file to confirm the passive port range on your server.
The solution to this problem is to ensure that the passive FTP port range opened in your server's firewall: