Air con only works when the car is cool

Hi all, hoping you can help me. We're experiencing a heatwave where I live and the car is pretty unbearable - especially for my poor kid in the back.

We have a VW Golf SV (2014). It's in the shade in the morning and the air con will work just fine. But in the afternoon it's in full sun, and the air con barely works - it's slightly cool and the fan hardly works either (even on max fan it's about the same speed as the normal lowest speed).

I did some research into the problem a while ago and remember that it might be a thermistor or resistor needs replacing, but I haven't been able to find what I found before again. And am sadly too broke to take the car anywhere to get looked at professionally. Any advice would be really appreciated, thanks!