I have a radio brick: http://www.alcatelonetouch.com/global-en/products/mobile_bro...

It doesn't work very well, but I think that's mostly A. bad indoor service on the network band I'm using it on, and B. lack of 4G.

When it does work, though, it basically does the same thing a smartphone does: serves a wi-fi hotspot, and acts as a 3G modem over USB.