Sounds like in BIOS you have it set for IGPU as the Main Display Adapter. Normally this BIOS setting is set on "Auto" that way when the motherboard senses a GPU Card is installed it automatically switches over to the GPU Card as the Main Display Adapter and disables the IGPU.
If in BIOS it is set to Auto try setting it on PCIe and see if it recognizes the GPU Card.
Also see if your your RX560 is showing up in Device Manager. Download the latest RX560 AMD Driver from here: https://www.amd.com/en/support/graphics/radeon-500-series/radeon-rx-500-series/radeon-rx-560
After installing the latest RX560 and still the GPU card is not working go to Device Manager and post any errors that shows up.
Also download a free program called SPECCY and see if your GPU card is showing up as populating the PCIe slot in your motherboard.