Fiberglass. Go to autozone or other like it, and get the resin, hardener, and glass matting. It maybe all in one package. Get some of the plastic/latex gloves to keep the stuff off of your hands. If you can get to the back of the crack, clean the area. Cut sections of the matting about 2 inch wide or more, and longer than the crack by about the same amount. Mix the resin and hardener in just enough volume for what you need. Soak three cut sections of matting in the resin. Layer the matting over the damage area. Use a spreader, roller, or gloved hand, and work all air out of the matting. Let it set a bit and harden.
Now go the from the front open the crack up a bit. Put some of the matting into the crack and fill it up. You may have to mix a bit more resin to do this, the old stuff will be hard as a rock. Fill the crack with fibers and resin sand it smooth, prime and get ready to paint. It' s really not too hard to do. Just be sure not to get resin on what you don't want it on. It will stay there.
If you can't get to the back side, just do the same thing on the front. It will be fine, but if you can layer the back it will be stronger.