Can't speak for the piece of kit that you have, and I'm not sure whether this is useful or not - but in the absence of any other ideas
What you could do is look up the base station location (unless you already know it) at Sitefinder.org.uk and then get in your car and take the thing along to the cell and see what the signal strength is then. That might help to narrow it down. On the other hand if it's a desktop that might be bulky to do.
And if it is a desktop, I'm not sure why you'd use an internal 3G card - case in point: here we get 5 to 6Mbps down with the modem outside the window, maybe a bit more, most of the time, with 50% signal strength.
If I put the modem down near the back of the computer - which is only about five feet away from where it is now - it would struggle to achieve half a meg and the signal strength would drop markedly. So given it's very position sensitive, a modem and USB lead (or router in the middle) is the way to go.