Did you try different short cables, PCIe slot boards and PCIe controllers with lower Gen3 spec?
Are the plugged cards showing some activity light? Is power supplied to the slot boards OK?
What does the seller say? May it be a fake board?
I've tried several cables I purchased recently (
amazon link). I only have 0.8m ones that seemed to work fine in a bifurcation card (although that has issues with negotiating gen 4 and I had to use gen 3 on them). I tried both pcie slots (the one linked to the cpu directly and the one behind the motherboard chip) and the card shows up fine, but still no gpus on the other side. The card shows green lights on all the leds besides the SYSTEM-FAIL one.
Regarding the gpus itself, the fans spin up and I know the power cables work since they work in the bifurcation setup. For the power being suplied to the pcie switch, I assume it would take from the motherboard, otherwise there's no way to provide additional power to it.
I can't say whether it's fake or not, it looks exactly like in the picture from
KingKaido on l1tech thread. The seller wasn't exactly helpful; they don't seem technical. I did contact them again to see whether they could help somehow (with firmware files or something).
What I've tried so far in bios: lowering the pcie slot to gen 3; pcie ari enumeration, acs, iommu.
It could definitely be the cables as well, since the pcie bifurcation card I have can't use gen 4 properly (in this case, I think the card is not working as expected since I can get gen 4 x8 [but not gen 4 x16] on rdna 4, while rdna 3 refuses to do gen 4 completely [can use 8x gen 3, but if I plug both cables in it falls to gen 1 x16]).
