The problem with fillers is the plastic of the bumper is flexible, it moves with heat, the car moving and people bumping into it. Filler does not do any of these things as it sets quite hard and therefore is not flexible. If it must be done it might be worth hevily scoring the plastic in and around the recess and fibreglassing it into and across the joint, then use a filler. The fibreglass may hold it stiff enough to protect the filler.
Otherwise get it to a bumper bar repairer who may be able to weld plastic into the recess and form a uniform material across the strip.

Cheers,

Justin P