in HD mode, it should be just +A~C not just any punch.
Not in HD mode, you have to do f+A, +C, f+D and yes you have to fairly low to the ground for the followup to work.
If you are getting the DM, it means that you arent doing the f properly (where you arent hitting the 'Back' part where when you do , it acts as x2 instead), or you doing the motion unnecessarily too fast. Slow down your motion of the f and the and you should get results.
Meanwhile, here is my latest vid
I also do his stun combo with inputs. You can kinda have a look at how its done. Funny enough, it registered as me NOT doing the 'back' in f motion, yet it still comes out. Haha, hypocrite I am