Airportable

Get the entire rear valance off the car. It’s a bit of a faff shifting the bolts but once off you’ll be able to push out the stoved in area & work on the inside of the damage to reenforce the area.
I’d be inclined to warm the area up, & with a piece of wood spreading the load, ease the lot back into shape.
I stress Warm, all you want is to soften the plastic a bit not melt it.
Melt it & you’ll be into a replacement bumper.
As had been said before there are many ways to skin a cat (assuming you would want to) but when faced with this type of damage, this would be my approach.
M