Ok, thanks for confirming the setup. This is not really a NAS, but more of a shared drive scenario.
I forgot to ask if you are even able to access the USB shared drive from macOS or whether it is just an issue of having an icon/link to it handy in the Finder.
On macOS you should be able to connect to this USB shared drive by clicking on the Finder's "Go" menu and select "Connect to Server....". Start by typing "smb://" and add the address name or IP address the router gives for the connected USB drive. It may even be possible to click the "Browse" button, but usually it is much easier to just use the address of the router & drive.
Sometimes it may be possible to browse the local network devices/drives within the Finder by clicking the Finder's "Go" menu and selecting the "Network" option which will list some local network devices found.
If you want to always have the shared drive mounted when you log in, then you need to add it to your Login Items. You do this by dragging & dropping the mounted network volume onto the Login Items section of the System Preferences/Settings. You may also need to enable "Connected Servers" in the Finder's General Preferences/Settings so it shows an icon on the Desktop. There is a separate option to configure what is shown on the Finder's sidebar as well.