If you do not care how it looks, epoxy / fiberglass the crack from the back (the crack will still be visible). Unfortunately, the paint on the front makes it hard to fuse from the front. But know this, no matter what you do, any contact on the front (or even air pressure from highway driving) will re-open the crack down the road. The only real solution is to replace the bumper cover (ugh!). Had the same problem on my last BMW. The dealer patched it from the back, sanded the front, and then used body filler and re-painted the front. Looked like new, until the lower part of the bumper cover brushed a curb stop in a parking lot, and immediately the crack was back. They ended up replacing the cover under warranty (it was a certifed used car and they did the repair before I bought it) since BMW said they should not have even tried that repair as it does not work. The downside of plastic bumper covers.