The Mi Box S 4k Android TV has been reported by some to have wifi connectivity issues. Here are the top solutions that resolved this problem for most user:
Doing a simply Factory reset could resolve your Mi Box S Wifi Connectivity issue.
5GHz Wifi Mode
Using a 5ghz compatible Router and setting it up to work in 5ghz mode also resolved wifi connectivity issues for Mi Box S users.
Changing Router Channel
For some strange reason the Mi Box S does not does not connect to some Router Channels that other devices can access easily. Changing your Router channel to 11 instead of it being on Auto also solves this problem. Please note that the Settings of each Router may vary so you should refer to the manual for clarity.