I am building a new rig, using an Asus Prime Z270-A mainboard plus 6 Gigabyte Radeon RX470 4GB video cards. However, so far only four cards are detected by Windows 10. When I connect the fifth one, it does not show in Device Manager (Windows 10 Uefi mode). Also, the fans on that card won't stop, what makes me suspect there is something wrong. I have above 4G enabled, also Gen 2 in the Bios. I changed the usb riser but still the same. I am using a SATA HD, don't know if that will be the issue since I have another rig with 6 GTX1070 using a solid state HD that was a breeze to setup.
Have you tested the fifth card independently? If you have a separate machine, try using it there, or try booting this rig with only that card, directly into the primary x16 slot.