PCIe 3x or 4x Splitters do not support Nvidia cards, are built primarily for AMD.
That's the first time I've heard of this..are you sure? I can't get mine to work on my Nvidia cards, so this would make sense.
Have you tried inserting your splitter in a way that the motherboard only has as many graphics cards as could be installed without it?
For example, if you have 4 physical PCIe slots on the motherboard, try putting one card directly in the primary x16 slot, then using a splitter card for up to 3 additional cards. This can help check if the splitter card can actually function at all in that board.