If, as you write, it will be over sftp, then why are you forwarding port 21? Port 21 is FTP, plus you need your NAT router to also be able to negotiate the data channel on a separate port.
If you don’t know you need this, then you may be in for a big surprise when you go down that rabbit hole and try to implement it.
It’s much easier to forward port 22 and use sftp. It’s also much more secure, particularly if you use ssh keys instead of passwords.
If, as you write, it will be over sftp, then why are you forwarding port 21? Port 21 is FTP, plus you need your NAT router to also be able to negotiate the data channel on a separate port.
If you don’t know you need this, then you may be in for a big surprise when you go down that rabbit hole and try to implement it.
It’s much easier to forward port 22 and use sftp. It’s also much more secure, particularly if you use ssh keys instead of passwords.