Unibeast USB not bootable after install

[nOd3N1AL]

So I managed to get the Unibeast USB installer to boot automatically without having to go into BIOS or change boot device - it came up automatically. I then proceeded to install Sierra on an external HDD and after the install (progress bar) the computer rebooted into Windows. Now I can't boot into either the HDD (which I presume has an unfinished macOS installation) or the USB I used for installation. I've tried everything in the BIOS and nothing's working. Any ideas?

FYI, here is the PC:

Alienware Alpha R1
Intel Core i5-4590T
12GB DDR3-1600 MHz RAM
NVIDIA GM107-based GPU 2GB (GTX 860M / GTX 750 Ti)

 

[nOd3N1AL]

OK I managed to get it working by disabling the Windows Boot Manager UEFI and inserting the USB just after boot.