Well if you use "cold weld" style glue, availlable at carquest, you can make anything stick togheter from plastic to metal, and believe me, it will stay in one piece, no matter what you do!!!!!
try 'dominion seal' two component plastik repair kit mix A part whit B part 50/50
let it dry then sand it like original filler, pretty much the same thing except it remain flexible ' harder to sand so try to apply it smooth..
eurethane also works well if its a "plastic" bumper, but always use plastic prep or silicon grease remover and sand with low grit paper werever you put mastic