That looks like brown coat (the sandy part) with a white coat over it. Just as a mention, I see no rock lath anywhere. Nevertheless, it is an easy fix. I see the brown coat has fell out from behind the edge of the white coat. Using anything to do the same around the rest of the perimeter, a pen, pencil or stiff wire to carefully scratch out enough of the brown coat so a small lip of white coat protrudes all around the hole. It doesn't have to be large, it just needs a small lip, even 1/8" over the brown coat. Brush all remaining loose sand out with a soft bristle brush.
If you have drywall mud, plop a big enough wad to fill the hole completely the first time, allowing it to squeeze out all around the hole, after that, remove the excess by pulling the putty knife, (which is I hope wide enough to span across the width of the hole, this is important) across the hole by starting at the center of the hole and pulling to the outside of the hole, doing this in all directions. Since the sand mix is so loose pulling across in one direction only will "peel" mud back out of the hole. Let it dry for a day or two, it is a lot of mud and it may even shrink and crack a little. If that holds, tape over it and finish off with a light skim over it all and sand smooth.
If you are using real plaster of paris like your post suggests you used at first, you will need to wet the hole after it has been brushed clean. A spray bottle will work for that. Dampen it enough to change the color of the sand mix, it won't take a lot, a few squeezes of the trigger at all angles to wet all sides will do. In the same way as the mud, do the same with the plaster, but this stuff sets up fast. After it is place with ooze everywhere, clean the knife off and keep it wet, do not use a dry knife over plaster, it is an entirely different beast than drywall mud. Still working from the center scrape off the excess with the wet blade, you may even be lucky enough to get it smooth enough not need any sanding. If you manage to succeed in that- congrats, it is an art in getting the right "touch" to finishing plaster. When it comes to small repairs in a plaster wall, using plaster of paris, by the pros, there is no tape needed just persistence of going over and over the hole with a sponge and wet knife to get an invisible repair.