Well, if you can get a half decent Signal outside, you could install a Mobile Repeater Booster system, they do work if installed properly, bottom line its an outside Aerial as high as possible on the House, ideally facing your best Signal Radio Mast, some decent cable feeding the Box of Tricks, then more decent cable feeding the internal Aerial(s).
Ive done a few of these installs in the past for people who are stuck, needs to be done properly as they key is to get as much isolation between the external and internal aerials, then choose your poison on the Box of tricks as to which Mobile Bands you want to cover.
Other option and probably cheaper is to install another form of Mobile Box that works through your internet Connection, but that generally will only boost Voice Calls.