It is possible that Dell 5090 oem Q570 chipset has 1x16 PCIe gen 4 lanes of cpu all wired to PCIe 4.0 x 16 graphics slot, and onboard M.2 slot connected to 4 PCIe 3.0 of chipset.  Dell only validated PCIe gen 3 nvme ssd in this model which may speak for that.  We have seen in the past Dell oem board m.2 is often curtailed to lower end spec (such as XPS 8900 M.2 was only x1).  If this is the case for 5090, you can try use a pcie4.0 nvme adapter to install the ssd in the x16 slot, if you do not currently have a dedicated graphics card in it already.
I have a Dell Optiplex 5090 MT desktop, I want to install a 512GB Samsung 980 Pro NVME GEN4 when tested with CrystalDiskInfo, I found it working at PCIe GEN3 Mode while the PCIe GEN4 Mode is supported! how can I enable the PCIe GEN4 mode from the machine to gain the maximum speed of this disk as now im working at half disk speed!
Adding Samsung 980 Pro NVME GEN4 to Optiplex 5090 MT