I think it has to do with the building, perhaps there is a Faraday cage caused by the copper net layer in the walls. From a similar experience, the doorstep of my apartment is really close to my other 2 neighbours (ground floor), and they all have WiFi routers above the entrance, inside — it was very hard to connect and use Wifi in the house on 2.4GHz due to the bandwidth, even if I changed the channels manually, so I was forced to buy a 5GHz router to circumvent the shortage. Also, my LTE doesn't work at all at my doorstep, so it's pretty apparent.