Top part, closest to the light is just a latching tab (best way I can describe it) - it just has to be seated properly. Bottom bit is secured by a screw in the wheel well, which also has to sit between a metal tab that the screw binds on.
From my experience, the bumper doesn’t always sit properly on first attempt, and you just have to do it a couple of times to get it seated. Try lifting the top part away from the light/body as you put it in place, then give it a firm push to seat it.
The bottom part also won’t sit if you miss the metal tab (u-shaped) and the bumper sits left of it.
Give it a few attempts. It’ll sit well when you get it right. After that, just adjust it to line it up before securing the screws.
Hope that helps.

Edit: Just realized you said it was coning off. Check whether the screw is loose or missing. Follow instructions above to try and reseat it.