From the looks of it, all you can probably use that USB port for is to connect it to the PC and surf in case your PC doesn't have an ethernet port. Similar to some modems like the D-Link DSL-502T which comes with a single ethernet port and a USB port. Not totally sure in your case because this thing has four ethernet ports, but that's what I suspect it is. I've never heard of BSNL or any other ISP in India giving out high-end devices with functionality like NAS, print servers, etc. If you post the model number, it'd be easier to figure it out. Can't really make it out in the first pic since it's a bit blurry.