Fiberglass it. I had to do the same thing for my bodykit when i broke a piece off. Try to snap it back in place and then use a fiberglass mat and resin on the back of the bumper, this would be the inside where no one an see it. Then bondo it on the outside and sand down.
Any body shop can do it, but its not a hard DIY. It takes a lot of time to get it nice.
You coul also cut it out and fill it the same way with fiberglass and bondo.
NEVER just use bondo, its not strong enough.