I have a 4" PVC pipe with a metal flange that originally appears to have been secured to the concrete floor with concrete nails. The nails have rusted through and out and the concrete is a bit broken up underneath the flange. Is there something I can "Inject" into the space under the flange where I would secure it with "Tapcons" or something else? Someone told me Epoxy, but I'm not sure how that would work, or if there is something better.

If the slots are intact then you may use Tapcons or plastic inserts with wood screws to secure the flange to the floor. If the cement is broken up under the flange, why not clean out the broken cement and chisel out any damaged cement left and grout with Sacrete patching cement. Let it set up and cure and then you can drill Tapcon pilot holes or install 1/4" plastic inserts through the grout into the original cement floor. Good luck, Tom