How does the portable device connect. If it has a ethernet port then it likely functions as a NAS and it will have a IP address you can connect to. Many times it just appears in the list of device you can connect to if you have microsoft file sharing enabled on your pc.
If it is some other type of connection like USB then the router must have a special feature that lets the router itself run as a NAS.
You are going to have to read the manual to find out how to connect a USB based drive to the router. Every router does this differently.