This thread is a list of Ethernet adapters that are known to work with the Xiaomi MiBox S 4K. Please don't clutter it with discussion unless you have something important to add.
Working USB Ethernet Adapter: Anker 3 USB / Ethernet hub, gigabit Ethernet, model AH212 (Amazon.com link)
MiBox version (from settings -> about) : Mibox4
MiBox Android version: 8.1
Android security patch level: May 5, 2019
Performance: Netflix app on the Mi Box reports 102Mbps, on my 100Mbps fiber internet connection.
If anyone adds USB ethernet adapters it might be useful to reuse this same format.