Wing FTP Software

When I try to connect to my wingftp server using
ftps://myftpserver.com
from an external location, I get the following errors in FileZilla...

Status: Connection established, initializing TLS...
Status: TLS connection established, waiting for welcome message...
Status: Logged in
Status: Retrieving directory listing...
Status: Server sent passive reply with unroutable address. Using server address instead.
Command: MLSD
Error: Connection timed out after 20 seconds of inactivity
Error: Failed to retrieve directory listing

myftpserver.com has a private IP and is behind a firewall. The firewall is NATing the public IP to private. When I attempt to make this connection from an external location, the firewall logs show's two entries
1. Allowed = (external IP) ftps/tcp 50875 990...
2. Denied = (external IP) 1048/tcp 50876 1048...

Is there a certain setting I need to add to the "General Settings/FTP Passive Mode" tab?
Or do I need to open something up on my firewall?

Yes, you need to forward all the passive data ports in your firewall, and please have a look at this KB article: https://bbs.wftpserver.com/viewtopic.php?t=5

That worked. Thank you.