For background task with timers to launch you must ensure that your app is added to the lock screen. You can request to be added as a lock screen application by adding the following code to your project:

var Background = Windows.ApplicationModel.Background;

var promise = Background.BackgroundExecutionManager.requestAccessAsync().then(
    function(result) {
        switch (result) {
            case Background.BackgroundAccessStatus.denied:
                // Background activity and updates for this app are disabled by the user. 
                break;
    
            case Background.BackgroundAccessStatus.allowedWithAlwaysOnRealTimeConnectivity:
                // Added to list of background apps.
                // Set up background tasks; can use the network connectivity broker.
                break;
    
            case Background.BackgroundAccessStatus.allowedMayUseActiveRealTimeConnectivity:
                // Added to list of background apps.
                // Set up background tasks; cannot use the network connectivity broker.
                break;
    
            case Background.BackgroundAccessStatus.unspecified:
                // The user didn't explicitly disable or enable access and updates. 
                break;
        }
    });

You can also check to see if your app is a lock screen app by using the following code:

var Background = Windows.ApplicationModel.Background;

var result = Background.BackgroundApplications.getAccessStatus();
switch (result) {
    case Background.BackgroundAccessStatus.denied:
        // Disabled by the user.
        // Cannot display on the lock screen. 
        break;
    
    case Background.BackgroundAccessStatus.allowedWithAlwaysOnRealTimeConnectivity:
        // Added to list of background applications.
        // Can display on the lock screen.
        // Can set up background tasks and use the network connectivity broker.
        break;
    
    case Background.BackgroundAccessStatus.allowedMayUseActiveRealTimeConnectivity:
        // Added to list of background applications.
        // Can display on the lock screen.
        // Can set up background tasks but cannot use the network connectivity broker.
        break;
    
    
    case Background.BackgroundAccessStatus.unspecified:
        // The user didn't explicitly disable or enable.
        break;
    }