Hi Riccardo @ricky_leaks ,
I am confused; how do you know that Face isn’t flat?
My guess is, that it actually is flat, but are getting some ‘Smooth Shading’ issues.
Try lowering (in the Z-Axis), the Edgeloops around all the buttons (I only highlighted a few here, but basically the ones from the first Inset):

That’ll change the Smoothing Angle and should improve the Shading…