Many FTP clients support AUTH (encryption of control channel) but do not support PROT (encryption of the data channel) - we require that FTP clients support both to ensure all data is transferred securely over TLS. If you are able to login but are unable to open a data connection and receive an error message similar to "Unable to open data connection" then lack of support for PROT in your FTP client is likely the cause.
You should attempt a connection using a more compliant client such as FileZilla, Transmit, Cyberduck, etc...