So? What's new? Our building got renewed about 6 years ago. At that point we found out exactly what they say. Cellphone signal was gone for two of the three operator available here. Reception was weak for the only available operator. Out at yard all operators gave full reception!
Reason? Thermal radiation insulation using very thin aluminium foil. It's not only in walls also windows are protected. Open window and placing phone just about 5 cm outside, full reception. Close window and place phone 5 cm inside, no reception at all. It's pretty clear situation.
Solution: Buying two mobile antennas and and about 20 cm of cable. Drill hole through the wall, Place another antenna outside, insert cable, place another antenna inside. That's it. Now you have passive pass through for the signal.
P.S. This solution works very well for cellars and basements too, if you want to have signal where it isn't normally available.