Mobile connectivity can break when WiFi goes out of reach In Android 6.0 the Internet connection is meant to be provided seamlessly by either the WiFi or the mobile network. Sometimes this breaks, when connected to the WiFi and moving away from the Hotspot (e.g. on a train) , the WiFi disconnects and the mobile network never takes over (despite full signal) . The only way to fix it is to toggle airplane mode on and off to reset the networking. Does anyone know a better work around? [Phone: Alcatel pixi 4]

I saw this also on Android 4.4 and phone Samsung Galaxy Note 2. But on 4.4, the seamless switching could be turned off in the settings while on 6 it's always enabled.
Currently, Android works in a way where it disconnects from mobile internet when you are on WiFi network. By doing so it saves battery because if on WiFi you wont be using the data connection anyway.

There is an option in Settings - Developer options where you can switch on "Mobile data always active" which should basically do what you ask. Keep the data connection on and on WiFi disconnect it will already be on and ready.

Note that this may drain your battery faster.