File sharing - WINDOWS 10 - ReadyNAS no longer connecting

One likely possibility - your old Duo uses SMB1 as the windows file sharing protocol.  This is a very old protocol, and not secure.  The vulnerabilities contributed to the WannaCry ransomware attacks last year, and Microsoft turned it off in response.

You can check for this, and re-enable it, by going into Programs and Features and then selecting Turn Windows features on or off.  Scroll down until you find a line called SMB 1.0/CIFS File Sharing Support  Expand it by clicking on the + sign and see if SMB 1.0/CIFS Client is checked.

If it is not checked, then you can restore access by checking it and clicking ok.  There is a security risk you are taking, basically it makes it easier for malware that reaches your home network to corrupt the files on your Duo.  

Another option (somewhat harder to set up) is to look for an NFS client in the windows features.  If there is one, you could also enable that, and also enable NFS file sharing in the Duo.

Current OS-6 ReadyNAS (for instance the RN212, which is the home NAS that is most similar to the Duo) support SMBv3, which is secure. Given the age of your Duo, you should reconsider upgrading.  You can repurpose your Duo as a backup NAS (which doesn't require SMB at all).  The RN212 is about ~4-5x faster than your duo, and supports much larger disk drives.

If SMB 1.0/CIFS Client is checked, the problem is most likely due to guest access (another area where Microsoft is making changes).  You can add a credential for the NAS in the windows credential manager to overcome this problem.  Just post back if you need more help on that.