A short while ago, we developed a dead zone on the same floor as our Wi-Fi modem. TV works fine in that room. It was always slow wifi there, but now nothing. Have tried both iPhone and iPad.
Do you have anything that emits its own wireless signal in that area of the home? Cameras, baby monitors, microwaves... Even some appliances that emit electromagnetic fields can kill WiFi like washers/dryers, dishwashers, fridge etc.
Most dead spots of WiFi are caused by WiFi disturbers nearby or the layout/construction of the home and location of the modem.