In general MindBell works well without being excluded from optimizing battery use on a standard Android. However, depending on model and vendor of your phone it might be necessary to exclude MindBell to ring regularly.
This is a description following some screenshots on an Android 7 device. Be aware that it might look very different depending on version, model and vendor of your phone.
(1) Phone settings
In your phone settings open the battery settings in the device section:
(2) Battery settings
Click on the three-dot-menu-button to open battery optimization settings (which are different from the battery saver mode):
(3) Battery optimization settings
This is where the battery button in the help dialog of MindBell should bring you to.
From here on show all apps not only the “not optimized” ones:
(4) Battery optimization for MindBell
Now exclude MindBell from optimizing battery use (optimization is enabled per default):
(5) Do not optimize battery use for MindBell
This is how it should look like if you followed the previous steps: