Man, I wish I could help. Other than "move house"
I'm not sure I've got much to suggest - you could try a directional 3G antenna, but your mileage may vary and you'll pay through the nose for a good one, I suspect.
It's not surprising that first line support were useless on this - you need to get through to someone technical. Preferably with a beard. Male or female, it doesn't matter.
Back to using your device as a native router... Have you dug around for drivers? Who makes this modem? Google isn't finding much other than links to the same question you've asked elsewhere! Have you had any success with the replacement device?