Wpf datagrid with combobox

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C#

WPF

I have a issue with DataGrid bound to ObservableCollection, where each column is a ComboBox. First ComboBox has ItemsSource binded to ObjectDataProvider. The other ComboBoxes are populated with items depending on the SelectedItem in first ComboBox. I'm using DataGridTemplateColumn\DataTemplate\ComboBox... The problem is that when I select an item from first ComboBox, SelectedItem is not set, because no items exist in the collection the DataGrid is bound to. DataGrid should create new row (add object to collection) as soon as I start selecting an item from first ComboBox. I can't add new row.

<DataGrid DockPanel.Dock="Top" ItemsSource="{Binding Path=Routes, Mode=TwoWay}"                  
                    AutoGenerateColumns="False"                                            
                    CanUserDeleteRows="True"
                    CanUserAddRows="True"    
                    IsSynchronizedWithCurrentItem="True"                    
                    GridLinesVisibility="All" 
                  >
            <DataGrid.Resources>                
                <ObjectDataProvider x:Key="Modules" ObjectType="{x:Type serv:PortPinDataService}" MethodName="GetModules"/>
            </DataGrid.Resources>
            <DataGrid.Columns>

                <DataGridTemplateColumn Header="Module" Width="*">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <ComboBox ItemsSource="{Binding Source={StaticResource Modules}}"
                                      DisplayMemberPath="Name"
                                      SelectedItem="{Binding Path=CurrentModule, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">                                
                            </ComboBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>                    
                </DataGridTemplateColumn>
                
                <DataGridComboBoxColumn Header="Module" ItemsSource="{Binding Source={StaticResource Modules}}"
                                        SelectedItemBinding="{Binding Path=CurrentModule, UpdateSourceTrigger=PropertyChanged}"
                                        Width="80"    
                                        DisplayMemberPath="Name"
                                        >
                    <DataGridComboBoxColumn.ElementStyle>
                        <Style TargetType="ComboBox">                            
                            <Setter Property="Width" Value="80"/>
                        </Style>
                    </DataGridComboBoxColumn.ElementStyle>
                </DataGridComboBoxColumn>                

                <DataGridTemplateColumn Header="Signal" Width="*">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <ComboBox ItemsSource="{Binding Path=SignalsByModule}" DisplayMemberPath="SignalName" SelectedItem="{Binding Path=CurrentSignalRegister, UpdateSourceTrigger=PropertyChanged}"></ComboBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>

                <DataGridTemplateColumn Header="Route to" Width="*">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <ComboBox ItemsSource="{Binding Path=Registers}" DisplayMemberPath="RegRouteInfo"></ComboBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>

            </DataGrid.Columns>
        </DataGrid>

So my question is why a new object is not added to DataGrid collection if using DataGridTemplateColumn?

What I have tried:

If I change the first column to DataGridComboBoxColumn a new item is added to collection as soon as I start picking an item and SelectedItem is set.