Those "strap on" boosters don't work at all at all. They are genuine snake oil. I mean, if a bit of placcy like that would improve things so much, wouldn't you expect Nokia and Ericsson etc to know about it and build it in?
If you live in a dead spot, the options are to change provider or to find the spots in the house that work. Upstairs might figure in that, or at the side of the house nearest the cellsite.
If you're really keen, go for an antenna on the roof with a good quality cable and a mounting in the room where you use your phone - effectively, the same as a car antenna kit. Then Bluetooth from the phone to your head. Or just leave the phone where it gets a signal and use Bluetooth anyway.
I use a contract SIM on O2 for normal, and have a T-Mobile PAYG in my old phone for those times when the O2 doesn't work. I only know of one or two places where T-Mobile's signal is better than O2's anyway - my lounge being one of them.