It is new. When i start my computer with the external power connected cant listen the bios sound, and when i disconnect the external power the computer starts all ok. But RX not work without external power.
Here a video: No funciona (Not working) RX 580 8GB DDR5 - Sentey - YouTube
Suggested/recommended psu watts is 550.
And powering modern cards off molex has long been NOT recommended.
Buy a new psu that has PCIe cable/s for the graphics card.