I had the same problem when I was trying to make my 2400G work with my RX580 to work. But fortunately I had the a BIOS option to disable iGPU on my Gigabyte AB350N.
Try manually downloading installing the latest chipset drivers available from AMD (it doesn't come packaged with the GPU drivers for the Vega 11 iGPU), then installing the latest Radeon GPU drivers for your Vega56. Obviously, use DDU first before doing any of these.
If you don't have in BIOS, you are out of luck i'm afraid. We need to wait for adrenalin driver for 2400G.