1) you have setup a user and password for that user. --- Yes
2) you have given that user access to either the whole disk or specific parts of it. --- Yes
3) you have CIFS/SMB assigned to that user. -- I have CIFS, AFP and FTP assigned.
4) your NAS has a fixed IP address on your network. --- no it does not so to access it outside of my network I use a dynamic DNS.
I did manage to get in the other day, im not sure what i did!!! But something worked but I have to keep using storage utility to create a link everytime.
Let me check a few things.
1) you have setup a user and password for that user.
2) you have given that user access to either the whole disk or specific parts of it.
3) you have CIFS/SMB assigned to that user.
4) your NAS has a fixed IP address on your network.
If that is the case then on your local network you should only need to use network places (or whatever win10 calls it) in windows explorer to be able to find your NAS (note it will most probably appear as a series of letters and numbers, this is normal). When you click on that you will be asked for the username and password you setup for access to your disk (see 1) above). Filling those in and clicking on remember password (if that is still in win10) should give you access to the NAS.
Note: there has been problems win the Microsoft implementation of the CIFS/SMB protocol in win10 that may or may not be fully fixed.