A possible solution if your mobile has a socket for an external antenna (some older models still do, orginally for using an external antenna on cars) is a high-gain external antenna.

These look like a mini TV aerial. You simply point them at the distant cellphone base station and they greatly improve the signal. These are absolutely legal and quite cheap, but hard to find. Of course you need to fix the antenna in place and the mobile then has a cable attached, but you'll have a good signal and can unplug the cable when leaving the house.

If this might be of interest, post back here and I'll search around for details.

RS