Putting the router in the modem's DMZ is only part of the deal. The DMZ only makes the modem invisible for traffic coming from the internet and will be directly sent to the router's WAN port. Before traffic can go to the NAS' FTP server, it has to pass the firewall of your router. Just set-up a port forward for TCP port 21 to your NAS' local IP address (extra advice: give the NAS either a static IP address or a static DHCP lease, so the address can't change, so the port forward can't suddenly go bad).
Before you do this, make sure your NAS is properly secured with a good username and password. Even better (depending if your router's firmware can do this): pick a non-standard outside port for the port forwarding and forward this to port 21 on your NAS. Something like port 45603 (just do something totally random, as long as you can remember it). Everytime you want to reach your FTP server, you can access it by:
»
ftp://
123.456.78.9:45603(The '123.456.78.9' you need to replace by your public IP address of course.)
The best thing would be that you have a static IP address from your ISP, otherwise you'll change IP every day and you have no clue how to log on if you don't have somebody at home to check your IP address.