Access a property of a child viewmodel

Question

Again, I'm confused about visibility or accessibility of WPF viewmodels.
I'm still at the beginning of my learning-curve, so please be patient with me ^^

My App is a main window with a menu-bar (Ribbon) and two child usercontrols, that are loaded with their own viewmodel.
One child contains a settingsVM and the other child contains a projectVM.

This is the Resource of the two children and their content-control:

<Window.Resources>
    <viewmodels:MainViewModel x:Key="MainVM" />
    <DataTemplate x:Name="settingsViewTemplate" DataType="{x:Type viewmodels:SettingsViewModel}">
        <views:SettingsView DataContext="{Binding SettingsVM, Source={StaticResource MainVM}}"/>
    </DataTemplate>
    <DataTemplate x:Name="projectsViewTemplate" DataType="{x:Type viewmodels:ProjectViewModel}">
        <views:ProjectView DataContext="{Binding ProjectVM, Source={StaticResource MainVM}}"/>
    </DataTemplate>
</Window.Resources>

<!--.... a lot of menu stuff-->

<ContentControl Content="{Binding SelectedViewModel, Source={StaticResource MainVM}}"/>

The correspponding Main Viewmodel:

namespace MyApp.ViewModels
{
  public class MainViewModel : BaseViewModel
  {
    //The BaseViewModel contains only the PropertyChanged-event.
    #region Private Properties
    private BaseViewModel _selectedViewModel;
    #endregion

    #region Public Properties
    public ProjectViewModel ProjectVM { get; set; }
    public SettingsViewModel SettingsVM { get; set; }
    public BaseViewModel SelectedViewModel {
        //This controls, which child is shown
        get { return _selectedViewModel; }
        set
        {
            _selectedViewModel = value;
            OnPropertyChanged("SelectedViewModel");
        }
    }
    #endregion

    #region Constructor
    public MainViewModel()
    {
        this.SettingsVM = new SettingsViewModel(this);
        this.ProjectVM = new ProjectViewModel(this) { Settings = SettingsVM };
        SelectedViewModel = SettingsVM;
    }
    #endregion
  }
}

Now I wanted to show a property of the SettingsVM inside the ribbonMenu. How can I access a property of the SettingsVM from the MainVM?

I tried several approaches like the following:

<TextBox Text="{Binding SettingsVM.Masterfile, Mode=OneWay}" />

or

<TextBox Text="{Binding MainVM.SettingsVM.Masterfile, Mode=OneWay}" />

but everything is empty. So can please someone explain to me, what the MainVM contains and how I could access it, or better its SettingsVM child? I thought I understood that but it seems not.

Thanks Carsten

EDIT

A piece of the mainwindow:

<ribbon:RibbonWindow
x:Class="PlcGenerator.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:ribbon="clr-namespace:System.Windows.Controls.Ribbon;assembly=System.Windows.Controls.Ribbon"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:i="http://schemas.microsoft.com/xaml/behaviors"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:MyApp"
xmlns:controls="clr-namespace:MyApp.Views.Controls"
xmlns:misc="clr-namespace:MyApp.Misc"
xmlns:viewmodels="clr-namespace:MyApp.ViewModels"
xmlns:views="clr-namespace:MyApp.Views"
mc:Ignorable="d"
Closing="ClosingApp"
Title="My App" Height="700" Width="1200">


<Window.Resources>
    <DataTemplate x:Name="settingsViewTemplate" DataType="{x:Type viewmodels:SettingsViewModel}">
        <views:SettingsView DataContext="{Binding SettingsVM}"/>
    </DataTemplate>
    <DataTemplate x:Name="projectsViewTemplate" DataType="{x:Type viewmodels:ProjectViewModel}">
        <views:ProjectView DataContext="{Binding ProjectVM}"/>
    </DataTemplate>
</Window.Resources>


<DockPanel LastChildFill="True">
    
    <ribbon:Ribbon DockPanel.Dock="Top">
        <Ribbon.QuickAccessToolBar>
            <RibbonQuickAccessToolBar>
                <RibbonButton Label="Speichern" SmallImageSource="Icons/saveHS.png"/>
                
            </RibbonQuickAccessToolBar>
        </Ribbon.QuickAccessToolBar>

        <Ribbon.ApplicationMenu>
            <RibbonApplicationMenu SmallImageSource="Icons/ApplicationMenu.png">

                <RibbonApplicationMenuItem Header="Neues Projekt" ImageSource="Icons/32x32/Add_32x32.png" Command="{Binding ProjectVM.NewProject}" />
                <RibbonApplicationMenuItem Header="Projekt öffnen" ImageSource="Icons/32x32/Scan_32x32.png" Command="{Binding ProjectVM.OpenProject}" />
                <RibbonApplicationMenuItem Header="Projekt speichern" ImageSource="Icons/32x32/Save_32x32.png" Command="{Binding ProjectVM.SaveProject}" />
                <RibbonApplicationMenuItem Header="Beenden" ImageSource="Icons/32x32/Exit_32x32.png" Click="MnuExitClick" />
                <RibbonApplicationMenu.FooterPaneContent>
                    <RibbonButton Click="MnuExitClick" Label="Exit" SmallImageSource="Icons/32x32/Exit_32x32.png" HorizontalAlignment="Right"/>
                </RibbonApplicationMenu.FooterPaneContent>
           
            </RibbonApplicationMenu>
        </Ribbon.ApplicationMenu>

        <Ribbon.HelpPaneContent>
            <RibbonButton Content="Hilfe" SmallImageSource="Icons/32x32/Help_32x32.png"/>
        </Ribbon.HelpPaneContent>
        
        <!-- Start -->
        <RibbonTab x:Name="StartTab" Header="Start" >
            <RibbonGroup Header="Projekt">
                <RibbonButton Label="Projekt" Command="{Binding ProjectVM.ShowProject}" LargeImageSource="/Icons/32x32/ColorLayers_32x32.png" />
                <RibbonButton Label="Neues Projekt" SmallImageSource="Icons/32x32/Add_32x32.png" Command="{Binding ProjectVM.NewProject}"/>
                <RibbonButton Label="Projekt öffnen" SmallImageSource="Icons/32x32/Scan_32x32.png" Command="{Binding ProjectVM.OpenProject}"/>
                <RibbonButton Label="Speichern" SmallImageSource="Icons/32x32/Save_32x32.png" Command="{Binding ProjectVM.SaveProject}"/>


<TextBox Text="{Binding SettingsVM.MasterFile, Mode=OneWay}" />


            </RibbonGroup>


    </ribbon:Ribbon>

        <!-- Fensterinhalt-->
    <ContentControl Content="{Binding SelectedViewModel}"/>
    <!--</Grid>-->
</DockPanel>

</ribbon:RibbonWindow>

Second Edit

Actually, I have no value inside the controls so I think the viewmodels are null...?
I removed all , Source={StaticResource MainVM} (above) and
<viewmodels:MainViewModel x:Key="MainVM" /> from Windows.Resources, as mentioned.

In <TextBox Text="{Binding SettingsVM.MasterFile, Mode=OneWay}" /> (nested inside the Ribbon-menu), I get a value so SettingsVMshould be loaded correctly.

SelectedViewModel should contain the selected viewmodel (including its Control), as the Control switches, when I change between ProjectVM and SettingsVM. I also could switch manually by Inserting <ContentControl Content="{Binding SettingsVM}"/> or <ContentControl Content="{Binding ProjectVM}"/>.

Mainviewmodel is initialized, the constructor is hit on startup.

Solved!

Thanks to @XAMlMAX and this explanation: WPF MVVM navigate views

My last fault was the wrong resource-declaration. I changed it to:

<Window.Resources>
    <DataTemplate DataType="{x:Type viewmodels:SettingsViewModel}">
        <views:SettingsView />
    </DataTemplate>
    <DataTemplate DataType="{x:Type viewmodels:ProjectViewModel}">
        <views:ProjectView />
    </DataTemplate>
</Window.Resources>

Now everything works as expected :)