The device does not go into Ready mode after Sleep and remains in Alarm

After the Sleep period is over, the device will try to update the home screen to go to Ready mode. If at that time the device faces a poor coverage of the GSM network, an error may occur and the status of the device will remain in Sleep mode.

This means that the device has returned to Ready mode, but is still displaying the previous status.
The status icon will be updated with the next alarm or periodic check.