Router has nothing to do with it. You don’t need to do port forwarding inside your own network. Port forwarding is to open up ports for external world to access your home network. Unless you know exactly what you do, you should never forward your ports or it’s just a matter of time that a hacker will sneak in and make damage.
Check that your shares have actually FTP service enabled. Log in to the web app using the administrator > Advanced Mode > Storage > Folder Shares
If your share(s) don’t have a checkmark in the FTP column, they don’t accept FTP requests. To enable them, click on the modify icon, check the FTP service for the share and click Submit.
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