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The riser cable might be causing the issue.
The riser cable shouldn’t hinder your 4090s speed; it might be because you’re using a Gen 3 PCIe version instead of a Gen 4 one.
Yeah, something is not right here. Gpu-z isn’t showing the correct info. Can you take the riser out?
You need additional drivers besides the GPU.
Currently it seems the risertube is faulty.
Many of these parts function only in PCIe 3 mode. Consider testing that setting first before attempting a replacement.
Have you verified all hardware connections by checking the Device Manager on the system?