I had a quick look to
https://bitcointalk.org/index.php?topic=2204115.msg22438508#msg22438508 and saw no mentioning of the above 4g decoding, which helped me out before to see the GPUs in device manager in windows;
“Above 4G Decoding” setting from “Disabled" to “Enabled”
Setting the PCI to Gen1 was mentioned I believe, these two settings are often ''the'' BIOS settings
Did you rule out the power? You could disconnect all other GPUs and connect one through primary slot for the monitor and the second on the M.2 slot.