Nothing like making something simple so difficult. Scrap DDNS and filezilla and all FTP client apps. This is simpler and works from anywhere there is an internet connection.
This is on a Linksys router, but it should be similar for most of them. It is only the menu layout in a browser that may differ.
Enter the router's browser setup (192.168.1.1 for Linksys) and, under Applications and Gaming/Port Forwarding, open port 21 as both start and end. Name it and click enable. Now, port 21 is open.
You now need your router's ip address. Look under status or just browse through until you find it.
Once you find it and you are not at home, type your router's ip address followed by :21.
ie: ftp://oo.oo.oo.oo:21 or try it without the :21. It will probably ask for a password, too, so you
If that gives you access, then you go to mapping at the top of windows explorer and follow the instructions, using the ip address as the link when asked. You now have a "hard drive" mapped to "My Computer". Simple and straight forward.
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DNS-323 FTP:
Follow the instructions on pages 34-35 of the 323s user manual for FTP setup.
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Following the above is how, if I recall the ip address access correctly, works the best. I tried the DDNS crap and it was impossible to access. Then, an IT co-worker gave me the info above and it worked within a couple of minutes of remotely setting up of my router. Look under administration in your router settings for remote access. Set a password and enable it. Then, when you are not at home, just type your IP address (or mapping my work, too) followed by :8080 for a Linksys router (it tells what port will be used). Now, it just takes some playing with all these settings on both devices for fine tune what I have given you here.