there are different ways to do it depending on the crack
for some, you can use a plastic welder and heat both sides and stick em back together
SEM makes a product called plastic bumper repair. it comes in an epoxy tube and you basically glue both sides back. this stuff also works like a flexible bondo, so you can smooth it all out too
you can always frankenstein it back together by drilling holes and using zip ties (preferred method due to easy
)