Do you have public IP address on router WAN?

I have exactly the same problem. I don't think it's related to the public IP adress. Or at least it shouldn't. My router doesn't have a public adress, just a simple internet adress, but the FTP access worked fine without public adress on my earlier router (which was a shitty Asus RT-N14U). With that I could connect to my HDD from anywhere, but with my WDR3600 it seems impossible. The strange part is, I can reach the FTP via the INTERNET ADRESS from LOCAL network. So that part works. I'm on my home network, I enter xxx.xxx.xxx.xxx:portnumber and I'm immediately logged in the FTP. If I try the same thing from anywhere else, I got denied. Internet access is enabled of course.
I think it's related to the problem, that the standard number 21 port didn't work at all, when I tried to reach the FTP via the internet adress. I set up a port forwarding that forwards a specific port from the internet to the internal number 21 port, and that made possible to reach FTP thru the internet adress, from HOME NETWORK. From outer network (which would be the important part) still the same result.

How To solve this problem