On the passenger side the molded bumper unsnapped from a plastic railing.
It has no damaged. It just un -poped. I wonder how to connect the bumper back
On the plastic bracket? Its about a 1" away from the bracket. But I don't see how
It connects??

I believe you have been "bumped" leaving no visible damage. There are claws on the plastic rails and they may be broken. They are, well... plastic!
Post a pic if possible. Helps to see this better.