Local automotive shop or hardware store should have fiberglass mat and resin with hardner. Tape up the front securely so it won’t separate while you’re working in the backside of it and nomresin will come through, clean up the area behind the crack, sand it, cut a piece of fiber cloth big enough to cover all sides of the crack, mix up the resin and hardner per instructions, use a disposable paint brush, brush some on the entire area you’ll cover, lay down the mat on it, dab on mor resin over the top until it’s all covered, air pockets are out of the mat and it’s entorely saturated. Let it cure, remove tape from front, reinstall and just leave it alone. Trying to fix the crack from the front will turn into an entire project so just leave it be, IMO...
