W880D4U Windows only displaying on iGPU

Make sure you have installed the latest drivers for your GPU. From what I can tell
looking at your board's manual and specs it would appear Resizable Bar is only
available when 64-bit PCIe Decoding is enabled (another name for above 4g decoding).
I suspect it is enabled by default on your board.

To troubleshoot:

Make sure you have disabled CSM
Disable the iGPU and Intel Onboard Graphics
Set primary display to PCIe

Then try to boot into Windows. The only available display adapter in this case
should be the B50 dedicated GPU, this may force windows to use it and allow you
to install drivers.

Good luck.

Did you try disabling the iGPU and ASPEED GPU in the BIOS? The options should be
under Advanced -> Chipset Configuration. You should also set "Primary Graphics Adapter"
to PCIe.

Failing that, I am shooting in the dark here, I have never used any of ASRock's
server/workstation boards (ASRock Rack).

You can try opening a support ticket:
https://event.asrockrack.com/tsd.asp

Server boards are built to a different standard for specific server workloads so
they quite often have major differences in their BIOS vs consumer class boards.