The mobile app automatically logs out due to authentication rule. This happens every 30 days. So, when this happen, you need to connect the device to a Wi-Fi network and log in with your username, password and service ID ( ARU).
For more information on how to download the app and log into your mobile device, please see our guide on: Downloading the Myprogress App
Below is a generic video demonstration of how to make the Samsung tablet keep you logged in: