Most mobile companies have plug in the wall signal boosters unfortunately Nokia does not seem to be one of them. Now not sure if it you go with the sim or the phone. They usually sell between £30 - £70.
My Brother in law has one for Vodaphone plugged in at his house and it worked ok using my old nokia and iPhone.
His is an old one about the size of a wireless router but they are much smaller now.
maybe have a google to see if you can find anything that will help you. They are plug in the wall but I am sure there must be ones that can connect to some sort of aerial.