You may be facing the problem that cannot access to FTP server after changing the default FTP port 21 to another port, cannot access from public. The error such as you did not have enough permission.
Actually, this is misconfigured on the Firewall, let’s try to get it fixed.
Open the IIS Manager, click the Server name under Start Page, double-click FTP Firewall Support, input the range 1025-65535 to the Data Channel Port Range.
Navigate to your FTP site > choose FTP Firewall Support, input your external IP address
Do not forget to configure on your Firewall to make a Network Address Translation (NAT) these port range also.
That’s it. Hope it helps and thanks to you for reading.