There is no evidence suggesting that hot and cold temperatures affect cell signal. However, some people experience signal issues when the temperature drops or increases.
The culprit is not the temperature, it’s the humidity in the air.
When it’s hot and humid, there is a lot of water vapor in the air. As mention earlier the water vapor can reflect, refract, and absorb the signal's energy, potentially causing your signal to be less than satisfactory.
On the other hand, extremely cold weather, or extremely hot weather, causes water vapor to freeze or evaporate. Remember, some moister in the air helps signal travel. During these extreme temperatures, the right quantity of moister is not present to help the signal travel to and from the cell phone towers. But, the lack of water vapor won’t drastically affect your cellular reception.