If you tap the "Home" icon once the game had already started, you will see a window asking you if you really want to leave the table. 

If you quit, or if you turn off the game, or if there was an operator error,  you will get a second chance to go back to the game. 

Thus, by returning to the home page, and if the game started has still not finished, you will be entitled to this message: 

Tap "Yes" to join the game in progress. Do not worry, during your absence, you were replaced by a robot. So you resume a normal game. 

If you type "No", and you start a new game, you permanently leave the game and lose your chips. 

To sum up: as long as you have not started a new game, and the one you left is still in progress, you can still join by tapping "Yes" when this screen appears.