I would say your gpu is dead in some way. You could still try the DDU, to see if it detects, but the fact that one works and one does not , even your card when i looked it up is a 300w tdp card, so if your 600W psu would be enough.
Try the DDU, but other than that i would say your card is defective in some way. Did you say you bought it off ebay or something?