Never seen so many ill informed people when it comes to bodywork....
1st thing.... BONDO IS GARBAGE!!!
2nd thing.... Bondo(or any other bodyfiller) is NOT to be used as a glue/bandaid for cracks!!! Bondo is a bodyfiller, its used to fill voids and low spots.
3M and Fusor make urethane repair adhesives that are amazing, Ive used several.
Here's what Id do...
Take the bumper off the car.
Grind down the damaged areas with a disc/angle/whatever grinder, 24-40 grit
Buy either 3M or Fusor body panel adhesive/urethane adhesive. (Fusor 147 is great, Ive bonded metal to urethane and fiberglass, and those 2 to each other)
Cut a support piece for the back side of the bumper where each crack is.
Apply body adhesive to support piece and bond to back of bumper.
Apply body adhesive to front side of bumper filling in the damage as best you can.
Grind down the front side of the bumper with a less aggressive disc 80-100 grit
Fill voids with 3M or Rage bodyfiller(BONDO brand is garbage, DO NOT buy it)
Do prep work to area... sand/prime/sand
DONT USE SH"T IN A SPRAY CAN!!! Do it the right way and get high build primer and automotive bc/cc. At best youre gonna have to respray basecoat over the fixed area and reclear the whole bumper. Id respray the whole bumper as long as its off the car.
Now the above process can get expensive if you only do it once or its your first time.... my suggestion to you is if youve never done bodywork and dont feel like spending unnecessary money, take it to a bodyshop so they can fix it the right way.