What you need to do is to set up 'port redirection' or 'passthrough' on your router so that ftp access requests to your external address is redirected to the internal address of the seagate.
I would point out that FTP uses two ports so you may need to redirect port 20 as well as port 21.
Also FTP is just secured by a username+password sent in plain text across the Internet.
If the seagate will accept sftp that is definitely preferred.
That runs on the ssh port of 22.