i have a asus prime Z270(with 7 GPUs) I also had the same problem.

try this:
must have windows 10 installed in UEFI Mode
update motherboards BIOS to the latest version and enabled 4G decoding.
if you already have windows 10 and drivers installed use DDU to uninstall AMD drivers.
now install any drivers 17.1.1 or any up to drivers 17.5.1.
Turn off your pc.

Install one video card at a time and Run atikmdag-patcher.exe.
Download the Patcher:  Pixel Clock patcher: https://www.monitortests.com/forum/Thread-AMD-ATI-Pixel-Clock-Patcher
If all limits are found, click "Yes" to patch and sign. If a limit is not found or if multiple matches are found, the patcher needs to be updated.
Reboot.

When you have done all the cards one by one, turn off your pc. and install all the cards.