You might also try a complete reset of the inetnet box.
(Note: Do a backup of your settings before or re-configure after)
I noticed a couple of times that accessing LAN devices via WLAN-connected devices is impossible despite the fact the firewall was even turned off. After a box reset the box started forwarding traffic between LAN and WLAN again.
To be honest I never used the NAS functionality but I can imagine that the same issue might also cause inability to access the NAS functionality from LAN and/or WLAN.
Also you can try if you can ping the internetbox-nas host. First to check if the IP address is resolved properly (also you might try to access the NAS application IP directly rather than using hostname). If ICMP ping towards the NAS IP works fine you can try to telnet (or use nc or any other tool) on port 445 (SMB/CIFS) to check if it is accessible.
From the description above it's not clear to me if you can reach the NAS device and just fail to access the shares or if you cannot even access the NAS service. My recommendation above is assuming you can't even reach the NAS service.
If you can access the NAS service on \\internetbox-nas (no share give should open the browsing of provided shares if you can reach the service) and you still fail to open any shares then I would expect it's an issue with the drives (filesystem, size or similar problem). Also assure your Internet-Box is on a recent firmware.