I've been surfing the forums for some answer and found out many people are having lots of trouble using the ftp server.
Since It's been so easy for me, and have it working with DDNS under my old (but strong) router dir-524UP, I think maybe It's because many are misstyping the adress.
How to type a DDNS ftp adress routed: ftp://|name|.|ddnserver|.com@|dns323ip| So I recommend setting for an static IP for your NAS.
Hope It might be usefull for someone