You can't "move" FTP. It requires ports 20 or 21 for console traffic (depending on active or passive FTP), and random high port numbers for data transfer. Since your ISP blocks the ports, you cannot offer FTP as a service.
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the ftp server accepts sftp commands normally, as well as standard ftp.
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Huh?
If you mean the built-in ftpd(8) server, it does not use the SSH protocol, used by sftp(1) and scp(1).
SFTP was designed to use similar end-user commands, but the underlying communication protocol is very different. And encrypted.