Imac ethernet port not working

I have imac made in 2018, the ethernet port is not working so I have to use wifi. I would like to use wired to router instead of wifi. In network preferences it says "cable not plugged in" but it IS plugged in. It worked fine for 2 years till a few weeks ago and just quit. Ive tried different cable, different port on router, etc. I have also tried plugging cable modem directly into imac (bypassing router) How can I tell if the port is actually bad or its just a software issue? I just upgraded to Big Sur, but still have same problem.