Hello Haemmiker,

In order to know exactly, you would have to briefly disclose your mainboard and the processor you are using because:
Processors provide a certain number of PCI-E lanes... if these are already used up by graphics cards, M.2 NVME and similar devices, then you have hardly any lanes left to address your network card... respectively your mainboard might downgrade one of your graphics cards to 8x to get some lanes free again. a random example from Intel ARK: https://ark.intel.com/content... Under "Expansion Options" you can see which PCI-E version and configurations are supported by the processor. with this processor you only have 16 full PCI-E v3 lanes available.
Furthermore, it depends on the PCI-E version and this is also displayed differently depending on the version... e.g.: Card with 4x PCI-E v3 lanes on a PCI-E V4 slot is displayed as 2x PCI-E depending on the mainboard because 2x PCI-E v4 = 4x PCI-E v3 etc...

Have you installed the latest BIOS on the mainboard? According to the specs, you have 4 x16 and 2 x1.