If your back glass is badly cracked, covering it with a layer of clear packing tape may allow the suction cup to adhere. Alternatively, very strong tape may be used instead of the suction cup. You can also superglue the suction cup to the broken screen.

If the edge of the glass is cracked, you can try to slide an opening pick underneath the crack and slowly chip the glass out.

Reapplying heat will help loosen adhesive and make the process easier.