are you literally trying to access from outside your home, or are you testing on a computer within your own network?
if testing within your own network, try testing from outside your network - as usually you could have some kind of NAT redundancy error...
disable the ddns feature of the dns-321 and only use the ddns feature of your router... as the only point of having the ddns feature of the dns-321 is if it is directly connected to the internet, without a router...
also - ping your ip address - from a computer outside of your network (such as a friend's house or a cellphone using cellular network, not your home network) - if successful ping your port 21 you have setup
if you are getting proxy errors then make sure if you have a real proxy running, configure it properly, if you have no proxy remove it or use "auto" for proxy settings on your client (computer/browser, etc..)
please let us know your results!
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and good luck!