Just yesterday I suddenly lost my Internet connection on my 2020 iMac 5K. I did all of the usual—reboot, boot into Safe Mode, restart modem, restart switch, change Ethernet cable, consider giving up—but when I discovered that other devices still had Internet, I searched online and found a similar problem reported 4 years ago that was caused by a firmware glitch. So I connected via Wi-Fi and downloaded the latest updates for Catalina. Ethernet works again. Problem solved.
I’ve posted this in case anyone else runs into the same problem.