It has to go under the old flange or else there is nothing to hold it down when you tighten the toilet to it. If it is in the concrete so the spanner will not go under it, then you may have to have a plumber remove the old one and install a new one, preferably one with a metal flange that will not warp like your plastic one did.
: I recently had to tighten the bolts because the toilet was moving, doing that did not help so we took the toilet off and noticed that when we tried to tighten the bolts, we probably damaged the flange. It is not broken but the area where the bolt is, is lifted a little so that the bolt would not stay in place. We tried to put a half metal flange on top of the pvc flange attached to the floor but when we put the toilet back in place, the toilet was still moving. What are we suppose to do. Where we supposed to put the metal half ring under the pvc flange and if so how since it is attached to the cement floor?