If you're looking for a USB ethernet that works, I've been testing a couple. I've tested three different USB 2.0 ethernet dongles that seemed to work only to stop working after a couple days. A reboot of the Mi Box would fix it but it would stop working again after a couple days.
https://www.amazon.com/gp/product/B00LLUEJFU/
This dongle has been going for a week now on two of my Mi Box's with zero issues with its connection. I only have an ethernet cable hooked up to it. No other connections. I suggest if you're going to add onto it you get the external power cable for a little more juice.
https://www.amazon.com/gp/product/B00QV5Q5HU/
Just helping anyone out with a Mi Box who wants a good ethernet connection.
Edit: If you're wondering what speed you can get with this dongle, I just did a speedtest on my Mi Box using speedtest.net. Sideloaded the APK. It's sideways on my TV due to not being Android TV compatible. Had to use a mouse to get it started:
https://i.imgsafe.org/e571a1e30d.jpg
I have 500/50 internet speed so this dongle took half of my download speed which is pretty good. More than enough for any 4k movie... even uncompressed.
I bought this one for my parents at Christmas and they haven't complained yet about issues, so I assume it works!
https://www.amazon.com/gp/product/B019GLIJ0Q