Start by drilling small holes through the fiberglass at the ends of the cracks. That'll stop the cracks from spreading. Then take a Dremel or similar tool and grind out a "V" along the length of the cracks. The size of the "V" depends on the size of the cracks and depth of the material. Put down some good masking tape about an inch from each side of the crack along the entire crack. Use fiberglass mat and tear it into little chunks. Mix resin and paint it into the cracks, lay down a layer of fiberglass, more resin, more fiberglass, until the crack is just a hair above the original bumper surface. Let it dry. Then it's time to sand it all down to be smooth with the original surface. Then follow up with a fiberglass reinforced body filler. That will fill in any pinholes that may be left over. Sand again. Prime, sand, paint, sand, paint. Done deal.
The above is just a typical way of repairing fiberglass cracks. The materials chosen will depend on some factors that are not provided in the question.