Latitude 5591 SSD in WWAN slot

Hello, I new here, just trying to experiment. I've seen that the latitude 5591 can have an SSD installed in the WWAN slot, and I've been trying to do this, but the system won't even start and go into the bios when I try. As soon as I remove it, the laptop boots up fine. I have an NVME in the M.2 slot, but was hoping to gain a little extra storage, and really just doing it for fun. Are there any settings I need to change in the bios to get this done? Will it need to be a PCIe x2 only? I figured the X4 might be backwards compatible, but it doesnt appear so in this case. Any information would be useful! TIA