Keep in mind that FTP and SFTP, despite the similar names, are completely unrelated protocols that work in completely different ways.
FTP is the one you’re probably more familiar with: it’s a typically unencrypted file transfer mechanism that can often be a bit tricky because of how it uses multiple ports.
SFTP is the SSH File Transfer Protocol and is basically an extension of SSH (Secure Shell), an encrypted way of remotely accessing a command console. It is typically built-in to most SSH servers. Major advantages are that it’s encrypted end-to-end, and that it only uses a single port (typically port 22).
Configuring these two things (FTP and SFTP) would never be the same setting in your NAS config, they would be configured separately. I don’t know what model of NAS you have, but make sure that you’ve enabled SFTP specifically, and that you’ve forwarded port 22 on your router. Also, make sure that you have not enabled remote SSH access on your router, because then that port will already be in use by the router and can’t be forwarded to your NAS.