Taking into consideration there are multiple apps that don't see your NAS, this sounds more like something tied to a network issue. If your S8 itself isn't detecting the presence of your NAS for whatever reason, you might want to focus on some Settings option in your S8 and/or your home network instead.
Assuming you've already restarted both your router and phone, if you go into your router's setup menu is there anything out of sorts with your S8 (as compared to your other devices)? If you have your NAS set up with an assigned IP address, try giving your S8 an assigned IP too. You might also want to try disabling IPv6 in your router's settings and see if that makes any difference.
On your S8, if you delve into its Settings menu, look around for anything relative to 'networking' and to 'sharing'. Samsung loves to add lots of tweaks and such so this could just be a matter of enabling some random option. Also, if you have any kind of firewall app check that, or if you have any anti-virus/anti-malware app try disabling all its features as a test, those kind of utilities are notoriously bad at creating connectivity quirks.