First, welcome to the forum!
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In actuality the software you are referring to is not necessary to access your DNS-321.
Do you have access to the web UI for your router? If so, log into your router and look at the networked device table (or some similar name), which will list all devices connected to the router. There you should see the IP address for your DNS-321 and all other connected network devices.. Next, open a web browser on a local PC connected directly to your router (wired/wireless) and enter each IP address listed in the networked device table until you find your DNS-321.
If this procedure doesn't work (and since the software you described did not find the DNS-321), a likely scenario is that the DNS-321 was setup with a static IP address that falls outside of the range of your current network. If so, you'll need to perform a factory reset of your DNS-321, which will delete all user settings, shares, accounts, etc., but should
not impact your hard drive configurations or data. -- To perform a factory reset: while the DNS-321 is powered up, press and hold the reset button on the rear of the unit for > 8 seconds. After the DNS-321 finished rebooting, the device IP address should appear in your router web UI as described above.