If it shows a generic error or "Unknown Device," right-click it and select . Go to the Details tab. Select Hardware Ids from the Property drop-down menu. Look for the DEV (Device) code: DEV_8139 means you have the legacy RTL8139 PCI card.
If an internet connection is active via another source (like a USB Wi-Fi adapter or tethered phone), Windows will pull a working legacy driver from its catalog. realtek rtl8139 driver 810x family pci-e gigabit windows 10
Scenario A: You Have a True Legacy RTL8139 / 810X Card (100 Mbps) If it shows a generic error or "Unknown
Expand if available, as older drivers are often hidden there. Method 2: Manual Installation via Microsoft Update Catalog realtek rtl8139 driver 810x family pci-e gigabit windows 10