I am experiencing some ethernet issues with board in question and opted to grab a cheap intel NIC.
According to the manual PCIEX16_3 is disabled by default and needs to be activated through BIOS.
Unfortunately I cannot find any settings relating to this particular slot under "Onboard Devices".
In a sub menu I can set its PCIE Gen to whatever I please but that is it. The BIOS search also only reveals this particular option when searching for "PCIEX16_3".
The NIC in question is currently not being picked up on by either Linux or Windows, suggesting the slot may be inactive. The card itself is at the very least receiving power though, as the LEDs blink when connected.
Probably missing something rather obvious here but alas there is little I can do about it.