Tesla K80 not showing up in HP DL580 G7

I have a HP Proliant DL580 G7 server with P65 BIOS update from 2015 (which I think is the latest BIOS).

I installed the PCIe expansion board in it and installed 4 x Tesla K80 GPUs. Connected them to the PSU board using 10-pin to 8-pin cable. When I turn the server on, I see the green lights on the GPU light up. In the BIOS I set PCIe to use Gen 2.0 setting.

Installed Windows Server 2012 R2 (with updates), and installed all drivers from the web. I don't have any devices in Device Manager that are missing drivers. However, Tesla K80 GPUs do not appear in device manager.

When I try installing the official NVidia Drivers, I get the error saying that "No compatible hardware found".

I also removed the newly installed PCIe Expansion board in case it was (somehow) not compatible to the mainboard, but that didn't help either.

What am I doing wrong?

Update 1: I installed VMWare ESXi with Enterprise License to see if Nvidia device showed up there so I could use passthrough and use the GPUs in a VM, but Nvidia devices did not show up in that list.

I installed Ubuntu 20.04 as it has the best hardware detection compared to other Linux distributions, but it failed to load GUI. Ubuntu isn't listed as one of the officially supported OSes for DL580 G7 so I moved on to the next option.

I installed RHEL 6.10 as it is one of the supported OSes, and when I did "lspci" I did not see any Nvidia devices.

Unfortunately, the bios on DL580 G7 does not support MMIO. The latest rom I got is P65 05/21/2018, but I still can not find MMIO support in it. Without MMIO, the system cannot map vRAM over 4GB, so, the driver can not recognize the device.