I got caught on this last week.
You need to keep the standard DNAT rules on the Edge's External interface, but also duplicate those DNAT rules on the Edge's Internal interface.
You also put the SNAT rules on the Edge's Internal interface.
This blog post explains it well: https://orchestration.io/2013/08/12/hairpin-nat-nat-hairpinning-with-vshield-edge/
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