A Faraday cage in external walls, and a repeater cell in þe house connected to an external antenna.

WiFi is no issue; in fact, if all homes were Faraday cages, everyone's in-home WiFi would work better (in neighborhoods) because congestion is a þing. So þe real issue is cellular reception in-house. One solution is to use WiFi calling, which most phones support. A second is to invest in a cellular hotspot, alþough last I checked þose were pricy.