Why does my mobile data turn off when I play Pokemon Go?
Solution 1:
Turn on flight mode, wait about 5 seconds, then Turn off flight mode, wait a couple more seconds.
This has worked for me several times in a row. Another fix is to start up the game while on Wifi, and then disconnect the Wifi in game. Although if you do this, the same disconnect will happen if you exit the game. So you have to continue playing.
Please also check this forum for more information.