Check out this video, particularly at the 3:40 mark. M.2 Mode & PCIe x4 Mode is displayed in the BIOS. It's a different Asus motherboard but the same general idea. https://www.youtube.com/watch?v=v7PEBscJbno
I see what you mean,
I think you found the problem but not the solution...
In my bios settings under advanced/on-board devices/ PCIE X4_3 bandwith
Im missing an option; it says X2 mode or x4 mode,
but I don't have the option for m.2 mode like in the video.
My bios is the latest version/ same with my m.2 / and windows.
I'm looking on asus if there's like a addon package to install on my bios,
I doubt it tho.