Repair a hole in plasterboard wall
Broken up plaster is not a good base for repair as it is springy, not stable and ridged.  You want to screw a piece if wood as a backing plate to give a good solid base to apply Fix-all.

Try to slide a piece of wood that is as wide as the narrowest openng, but 6" longer than the area you want to repair.  You will span the entire area from inside the hole and then secure the wood with screws through the front.  Drill a hole in the middle of the wood for a finger-hold, or drive in a screw to hold onto.

Apply Fix-all to the hole and smooth to just below the surface.  Fix-all is not very forgiving, so you don't want to end up with too much material and the need to try to sand it down to level with the wall surface.  Better to apply it shallow and fill it out with spackel and then finish it smooth.