So you have tried everything? Like the connection to the motherboard is properly seated, the power cable have been properly placed, and you have plugged the video cable to the card. Maybe you have a DOA GPU? Have you also tried to update the bios as well? Since you have older CPU, try doing a bios update.
I would return it to the store, and ask for a replacement, or a new brand, good idea to go with Sapphire IMO