You have to be in the same workgroup as the devices you're looking for. The SSID of the router CAN be the name of the workgroup but doesn't have to be. Each device has its own workgroup settings even if on the same physical network.
And is SMB turned on in your NAS?
True, you don't need UPnP service, just use the file browser. That's what I do because I can delete and copy files, not so with UPnP.
You can access the NAS via the 604 Opera Browser by turning on http service on the NAS, this is much faster than using the file browser, you just set the ip address of the NAS share as a favorite or bookmark (just like any other web page).
i have a NAS running with upnp turned on in the network... anyway i heard it can just connect via smb. but smb is also turned on on the NAS.