Mi Box 3 stopped connecting to android remote app on phone out of blue. I found the fix searching from various thread and thought of listing down, things to do before factory reset . I had issue with connecting the remote and either of the following step help me get back the connection

Here is list of thing to try and connect after each step

1. Cast YouTube video from any phone. Probably Cast service wakes up sleeping remote service 😃
2. Clear cache/Data of Android Remote app
3. Restart your phone
4. Restart Andoird Tv box
5. Android Tv : Setting -> Apps -> Android Tv Remote service - Clear Data
6. Android Tv: Setting -> Apps -> Bluetooth/BluetoothRemote - Clear Data
7. Change your WiFi security from WPA2 to WPA
8. Change your WiFi operating frequency from 5GHz to 2.4GHz

Point no 5 is strange but last time this was the culprit. Hope it helps.

Feel feel to add more way to get the remote back.

Here I found point no 5 which helped me just before I was going to factory reset.