you have to use RebarUEFI.
if you have any other questions you can start here or github.
I’ve already modified your bios and for flashing method.
ROG-STRIX-X99-2101-RebarUEFI.zip (7.3 MB)(UEFIPatch + Microcode update)
This requires activating above 4g decoding and disable CSM first, if this setting is not visible in the bios you need to use RU.EFI + tutorial YT.
NOTE : RebarUEFI v0.3