Not sure exactly what the issue is. Can you provide pictures of the top and sides. Why am I asking this, well if the backplate is not fully seated at the bottom, you will have a difficult time releasing it as it is wedged in.
Make sure it is fully seated at the bottom and top. Then try the pin and push it down through the hole, it should then release down so you can pull the top out.
Having taken the little cover off, over the spring mechanism, it is hard to see what pushing that spring does…??
Looking inside the unit, it doesn’t appear to be moving anything which could lock it, when pushing down…??