If you use Android 4.1 and higher:
Settings > Device > Application Manager > C.A.T.S. > Show Notifications ON/OFF

If you use Android 5.0 and higher:
Settings > Apps > C.A.T.S. > Show Notifications ON/OFF

If you declined to receive notifications in the first place, try to reinstall the game and agree to get notifications in the tutorial. Do not be afraid; you will not lose your progress if you follow these steps:

1. Write down your Player ID (i.e. John#12345) somewhere so our Support team can restore account manually if something goes wrong.
2. Make sure you are logged into your Google account, and Cloud synchronization is turned on in the Options before deleting the game. 
3. Delete and reinstall C.A.T.S. from Google Play Store. 
4. Launch it and agree to notifications in the tutorial. 
5. Navigate to Options and turn on Cloud synchronization. 
6. You will see a pop-up asking whether you want to restore your progress. Tap "Yes."