You haven't made it easy for yourself by using a stainless steel mesh and the Polypropolene the bumper is made out of is notorious for not being reseptive to adhesives BUT go along to B&Q and go to the adhesives section, look for some of this, sounds and looks a bit gimicky but OMG does it ever stick :shock:
get a couple of tubes of it and use plenty of it, before you do though,with a Stanley knife, scar up the inside of the bumper (just where you want to stick the mesh) to act as a key for the adhesive, it dries white and is flexible, waterproof and very sticky. Make sure you don’t get any showing round the edges of the bumper cut out. Once dry spray over with some Halfords satin black spray paint. Leave it over night to dry somewhere warm. Cut all the OEM clips off the inside of the bumper to make it as smooth as possible on the inside then offer up the bumper to the car, whilst standing there holding the bumper in position you will notice there's a lip on the top inside edge of the bumper that sits under the thin strip/body panel (the bit that sits under the head lights), I drilled 2 x 3mm holes about 10 inches apart and used 2 stainless short self tapping screws to fix the top of the plastic bumper to the metal bumper support, with these in place and the 2 bolts that sit either end of the bumper in the wheel arch, you should find that fixes the bumper into position nicely.
Hope that helps
Steve