Some questions. What is outside high temperature? What is your supply temperature upstairs at several locations? Your furnace or air handler is located in the attic? to be clear, where is your thermostat located? What size is your AC? Same as one replaced?

TD

If you don’t want another company to look at it because you will have to pay a service charge, you will have to have the installer come back out. It is a new unit, under warranty. Call them . In our area we have been in a heat wave of near 100 degree temperatures and it is hard for the AC to maintain temperature. I don’t understand your reluctance to call for service on a new install. You can void the warranty by doing something to it yourself or having a third party adjust it. Call the installer.

I don’t know if this will help but have you looked at the schedule setting in the nest app? I would set my nest for 70 on the actual wall unit. Id go downstairs to work and there would be times I would come upstairs and wondered why it felt a bit warm. I would look at the thermostat and it would now be set at 72. No one changed it. I went into the schedule for each day. It shows a blue dot with the different temperatures at different times of the day. I went and removed all that said 72. That did the trick. Maybe see if any if those blue dots say 74 for the times you are asleep.