Connections to the server are timed out.
To solve this issue the NAT ALG feature needs to be enabled on the USG. To accomplish this you neeed to configure detect ftp command in teh firewall interzone policy view and enable nat alg globally by using command firewall alg-detect enable in system-view.
The FTP service doesn't work through the NAT gateway because ftp is a multi-channel protocol, This being the case, teh NAT gateway cannot translate teh same private/public ip address pare for 2 diffrent port numbers (20,21).
Always use the NAT ALG feature when having to configure multi-channel protocols throuhg a NAT gateway. It's the only way to make these services work properlyw.