Qatar Airways runs an IATA approved DGR course that is very comprehensive and recognized globally thru IATA.
The course cost is $1000.00 USD and is about 7 days including testing. Well worth it.
As for the other stuff.... the best certification out there that is recognized (again) globally is the IATA/FIATA certification directly through IATA. You can check their website. It covers all aspects.

Good luck.