still clinging to my x99 system. You helped me with a modded bios 3 years ago. Back then the rebar mod was not available and perhaps there is new microcode.
You will have to flash this bios with FPT or USB Flashback method, enable 4G Decoding in BIOS settings, disabling CSM and boot in full UEFI mode.
Then run RebarState.exe as admin and put 32 as value. Reboot again your computer and you should see Resizable Bar “Enabled” status under GPU-Z.