NFHS: this cannot be UNS, as it involved contact. It is a PF for UNR.
The basic spot is the end of the run, which is the GL. The enforcement spot depends on whether it is live-ball or dead-ball. If live-ball, we apply ABO: a foul by the offense behind the basic spot is enforced from the spot of the foul. If dead-ball, the basic spot is the succeeding spot. (This seems to be your main question.)
If live-ball, no score, go back 15 from the spot of the foul, assess whether that's sufficient for a first down and otherwise replay the down, clock on the snap. If dead-ball, the score counts, B can choose to enforce the 15 yard penalty on the try or the kickoff.
As TXMike suggests, this should be ruled a DB/PF unless it is abundantly evident that the runner had not yet scored when the foul took place (say, the runner was still well outside the 10 YL). When in doubt, the score counts.