you aint holding that with just fibreglass.
you need to get the bumper secure and in place before you even look at fibreglass.
you need to make yourself some little rectangle strips out of some old plastic or metal, drill a hole through either end, put the bumper cracks up against each other, drill through the holes. you can then either use self tapping screws or pop rivets. but before you screw or rivet it, use a big drill bit to drill into the bumper slightly to countersink your screwhead/rivet.
when its all in place and your bumper is all lined up and not hanging off, get a grinder and grind all your cracks, it should pretty much melt together, grind it well.
then on the back and front you wanna scuff it all up with p80 and skim the front and back with fibreglass resin (fibreglass in a tin that you mix with hardener(the sheets are a load of b****cks imo, too messy for my liking) you can use the sheets if you like but they aint as solid as the stuff in the tin.
let that dry, grind the front down so its level, then filler it and block as normal