Ethernet Internet Connection Unreliable with Catalina

It was no easy task, but I set up my home network so that I would be able to connect via ethernet from my 2nd floor home office even though my modem & router are downstairs. For some reason since updating to Catalina, the LAN connection is extremely unreliable, and more often than not... actually it doesn't seem to be intermittent anymore - it is now always showing a yellow light & says "USB 10/100/1000 LAN has a self-assigned IP address and will not be able to connect to the Internet".

I've tried a number of troubleshooting steps, but none have had any effect.

I have an older Mac Mini connected in this same fashion with the exact same settings, the only difference being that the Mini is running High Sierra and doesn't have any problem connecting via ethernet.

Any suggestions welcome & appreciated.