Network path was not found

Have you tried using the IP address in place of the NAS name?

Even though it is not a name resolution problem (as I can map drives if I first specify the username), I did try specifying the IP address and also the FQDN, just in case the Windows system had something cached for the name (although the problem with the bad NAS appears to occur with whatever Windows Server 2016 and 2019 system that I try). I am in the process of comparing all of the configuration files that are in human readable text format to see if I can find any difference between the 2 NAS units that may be causing the problem with one of the units.

After running a script to compare all of the configuration files between the good and bad NAS, I couldn't see any configuration differences that would cause one the behavior that I saw on the bad NAS. As a workaround I ended up enabling AD integration on the bad NAS, and Windows 2016 and 2019 can now see it normally again, and my backup software can access it correctly. I would prefer to have AD integration on this particular NAS anyway. It previously had AD integration, but I removed it to enable cloud access and then removed cloud access to see if that was causing the problem (as I no longer required cloud access for that NAS). I assume that adding either AD integration or cloud access made some configuration change that wasn't completely undone during the removal process. Even though I plan on keeping AD integration enabled for this NAS, I still would like to figure out why it behaves different from my other ReadyNAS 2312. For now, I will just be cautious about adding AD integration to any of my Netgear NAS units if I plan to remove it again later.

If you need remote access again, you can look into VPN options.  I personally like ZeroTier, but I'm not using it in a business environment.  Other options implemented via your router/Internet gateway may be better for your situation.