Sorry to hear that you have this problem.
It is not a feature of 'cheap' - it is often down to the building and the local mast. Years ago, I lived not far from the O2 mast that supplied my signal. Unfortunately, the house was in a slight dip and so I could only get a signal by running upstairs.
This can happen with any network and if you previously had a good signal with another network, it may be that was the only appropriate network for you - assuming that you use your phone mostly at home.
