Are you talking exclusively about 5G coverage? If so, mmWave (the 5G tech that VZW uses) is notoriously bad at penetrating walls, so that probably explains why you need a window to get reception. see here: https://www.digitaltrends.com/mobile/verizon-5g/#dt-heading-verizon-5g-network-coverage-map

I assume you can enable non-5G service on your phone, which is where I would start.