Your choices here are a PCIe (or PCI if this is a really old PC) network adapter card, or a USB Ethernet adapter. USB is easier since you don't need to install an internal card, but will limit your network speed if your PC only has USB 2.0 ports. If you have USB 3.0 ports (and get a USB 3.0 Ethernet adapter), performance should be pretty close to that of an internal card.
PCIe: https://www.amazon.com/UGREEN-Ethernet- ... 078GJ1594/
Legacy PCI: https://www.amazon.com/dp/B004F3CO5M/
USB 3.0: https://www.amazon.com/AmazonBasics-100 ... 00M77HMU0/
Before buying a new adapter for the PC, you ought to make sure the problem isn't on the router side. Can you plug another PC or laptop into the router port to make sure it is functional? (If testing with a laptop, make sure the laptop's WiFi is disabled to ensure you're really testing the wired port.)