In motherboard's that support APU processors (Processors with Integrated Graphics) in BIOS there is a setting that needs to be put either in "Auto" or "PCIe" or something similar in nomenclature.
If the BIOS setting is set on "Auto" when you installed the GPU Card it should have automatically detected the GPU Card and switched from Integrated to Discrete GPU Card and make it the Main Display device.
But if in BIOS it is set for Integrated Graphics as it Main Display Device then it won't switch over to the new GPU Card.
If BIOS setting IS SET at "Auto" or "PCIe" or something similar then you need to troubleshoot further to see if the Motherboard is defective or the GPU card is defective or not installed correctly.
Install the GPU card on another PCIe slot and see if it is recognized under Device Manager - Display. Also see if there any errors showing.
Make sure you have the 6 Pin PCIe GPU Power cable connected to the GPU card if applicable.
