Those plastic seams have little tabs (as you see/know) that must catch to hold. If enough tabs are still good, it should pop & hold, but you have to get the cover loose in other areas to manipulate. Take a good look at where these parts are secured & find the bumper cover screws/tabs. Remove them on bottom & then pop it into place...you might end up taking the entire cover off...
If too many tabs are broken, go to a salvage yard & find a replacement (same color, hopefully), or time to add a sweet rear ground effects to your ride. Good luck!