Why can't Ring Central connect to my SFTP for Archiver?

Has anyone gotten his working? Ring Central simply says “You got Disconnected”

The Firewall has a Port Forward rule setup.
IPs indicated by Ring Central are part of the whitelist group allowed on that Port Forward.

The QNAP has the QFTP app running.

Bonus - I can connect from my home (when I whitelist my home IP) to the QFTP server.

I have confirmed QFTP is using a current cipher:

debug2: KEX algorithms: curve25519-sha256, [removed by moderator] ,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,ext-info-c, [removed by moderator]