First I would suggest that you only have one device updating the DDNS server - preferably the router.
Next - get the ftp server set up and tested using local access - ie. from the local network ...
Step by step
1) Set the DNS-321 for a static ip address, make sure to enter a gateway address.
2) Create the user
3) Create the folder
4) Give the user access to the folder
5) Start the ftp server
6) Test using local access.
If there is a problem at this stage, fix it before moving on - once local access works
7) At the router - forward port 21 to the static address of the DNS-321
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Test from outside the network.
For the test in #8, you might want to make a note of the router's WAN or public ip address and try that if access using the DNS name does not work and, depending on the ftp client you are using, it may be necessary to configure for active ftp.