Whats the language you use on your Mi Box S? I had the same "issue". Found out that it doesn't yet support my language (Dutch) yet. Although I've been able to use it on other devices just find just not on Android TV/Mi Box..
The following languages are currently supported with Google Assistant on Android TV:
On Android TVs, the Google Assistant is available in English, French, German, Hindi, Indonesian, Italian, Japanese, Korean, Portuguese (Brazilian), Spanish, Swedish, and Vietnamese. More languages are coming soon.
As mentioned here: https://support.google.com/assistant/answer/7394513
it works in my case, with the remote, and recently they fixed in google home hub, so now works fine with mi box S
try this:
-first Check if you have your Netflix is linked to your google account (in “google home app”/ assitant/ services / video. ( obviously use the same google and netflix account in the mi box) maybe This fix, but if not....
-update your mibox, and then update the Google assistant app and chromecast app in the google store Or by configuration options (sometimes you have to uninstall updates and reinstall the updates to work properly) Many people fix their problems doing his
-add mi box to your google home appliances in google home app.
pd. Just in case you want to use a google Home mini Assistant Or similar you have to set up the Mi box as default tv player For that google assitant device.
i hope this help you