I have Sapphire RX 470 Nitro 8gb working by using Gigamaxx method.
My main monitor is LG ultrawide with 2 hdmi input.
For initial setup, I used a second old LCD with VGA port connected to motherboard VGA port. My main monitor was connected to Sapphire using hdmi. Primary gpu was set as igpu in bios.
Once everything worked in order and I got accelerated display, shutdown the system. Pulled the connector from second moitor put a VGA to HDMI converter dongle onto vga connector and plugged it to the second hdmi port on my main monitor. put back my second monitor into the closet.
Now, everytime I boot, it is all black screen until login screen appear.