For autosync (including instant upload) to work apps must run in the background. If manual sync works but autosync does not, your device is probably kiling background apps. On devices from Xiaomi running their MIUI firmware you need to change some settings.
Open the phone system settings, go to Manage apps
find our app (OneSync, DriveSync, Dropsync,..) in the list, then turn its Autostart switch to on, and set Battery saver to No restrictions
The Autostart flag can also be turned on/off under System settings > Permissions > Autostart