@Damo,
With respect to port scanning, you have to install or use a command like tool like nmap.
From your answer(s), I can safely deduce that it is no problem to make a local connection to the NAS.
That does not imply that "outside" connections can be made: any FTP connection to the NAS is an "outside" connection AND these "outside" connections will be blocked, if the firewall (on the NAS) has been enabled and the appropriate ports are not allowed in the firewall.
In essence, all of the above is normal behaviour for a NAS (Network Attached Storage), which is a storage component on a local network.
The reason why SFTP connections do work is the fact that they are (more or less) "local" by nature (simply stated).
In short, you are wanting to "force" an "outside" connection (i.e. FTP of some kind), even though the NAS is on the local network.
The conclusion should be that you simply do not need the FTP/FTPS connection (i.e. you can use SFTP with Plesk) or that you can do a "mount" (i.e. allowing backups on a mounted share, that will be treated as if you are backing up to the server repository).
Personally, I would strongly recommend to leave the desire for FTP/FTPS for what is and use a SFTP or a "mount" approach to store your backups on the NAS.
Hope the above helps.
Regards....