Replace GPU card, 4 led lights on but black screen 2008 20" iMac

Hello All,

Really need all your precious advice, I got an old 2008 20 inches iMac, GPU card died and order ATI 2600 for replacement, card came yesterday and excited to do the installation. After the installation and ready to boot up, problem comes. Have chime and 4 LED lights all on but black screen. Don’t think the card is faulty, is it wrong EFI firmware issue? If I have the black screen how could I download the update? Connect the imac to other monitor works? Thanks for all your advice and kind of frustrated right now, appreciated.