Can't get RTL8153 Gigabit Ethernet Adapter USB 3.0 working on Ubuntu 20.04
Solution 1:
From the comments...
It looks like the interface is coming up at only 10Mbit/s, which might indicate a cabling problem, or a problem with a hub/switch/router. Are you using cat 5e or cat 6 cables?
Update #1: replacing the ethernet cable fixed the problem.