It doesn’t really sound like a network issue. And I am not a Mac guy so I don’t even know what some of the words you are using are. However, assuming you are being asked to login to the NAS from this app then you are probably goosed unless you know the details. I recommend a password manager in future.
In your position, assuming I could see all the data on the NAS on the MAC, I’d be tempted to copy it all off and then reset the device following whatever process the manufacturer recommends. Then start again and remember the username/password this time.
Just an idea to consider, hopefully someone with an even brighter one will be along soon!