Opening Second Port for NAS FTP Server

Running through an Asus RT-N12D1 router with the FTP server set up in a LenovoEMC PX4-400R NAS, I cannot seem to get access to it. The Lenovo has no set up choices for the FTP server nor any details of what ports it needs so it’s either enabled or disabled. The router has port 21 open and the client is in Passive mode but I understand that the router requires a second port to be open. What is that port?

The error in the FTP client is that the Connect Port failed.