You got anything else that can connect with Ethernet, game consoles, smart tv , different , powerline type adapters, laptop/pc etc ….if you can connect other devices to the router with WiFi, but none of the Ethernet ports on the router work with your desktop, then either the router is faulty, ( in that it’s Ethernet connectivity is not working ) although its other functions are apparently OK , or your desktop is faulty ( TBH it’s much more likely to be the desktop, either hardware or configuration issues ) you really need to establish which it is .
If you haven’t already done so , reset the router to factory settings , but you really need another Ethernet device to try to prove to the router or your desktop .
You could try setting up a PPPoE session on your desktop, this entails you connecting your desktop directly to the ONT with an Ethernet cable , therefore eliminating the router from the setup, if this works you basically have proved the router to be at fault, but apart from the temporary physical relocation of the desktop to be near the ONT , you also need to be able to configure the desktop to do this , probably easier to use a different Ethernet device
