Change keyboard focus to a specific control - wpf

Question

I have a usercontrol which has several elements at different logical level. By default, keyboard focus goes to logical control at highest level, but I want to focus to switch to some other control which is deep down in logical tree (but all the controls are loaded when this user control is loaded).
The XAML is shown below, when the OverlayContent (a dialog) goes away, I want focus to move to "EigthLevelTB" a TextBox, but by default control goes to "ThridLevelGB". User can tab and navigate to "EigthLevelTB", but they want default focus to be here rather than tabbing.
I have tried setting focus on this EigthLevelTB in UserControl.Loaded, FocuableChanged events, etc. Also I tried setting KeyboardNavigation.TabIndex="0", but nothing is working for me.
Please suggest what could be going wrong here?
<Grid Name="OverlayGrid">
<Grid Name="TopMostGrid" Grid.Row="1" Margin="1,0,1,0" >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid Name="FirstLevelGrid" Grid.Column="0" Grid.Row="0" Margin="0,0,0,0" Background="Transparent">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<ContentControl Grid.Row="0" Margin="5,10,5,10" Content="{Binding HeaderInfo}" KeyboardNavigation.TabNavigation="None" Focusable="False"/>
<Grid Name="SecondLevelGrid" Grid.Row="1" Margin="1,0,0,0">
<Border Name="ThridLevelBorder" Margin="5,2.5,5,2.5">
<Border Name="FourthLevelBorder">
<Grid Name="FifthLevelGrid">
<Grid Name="SixthLevelGrid" Grid.Column="0" Visibility="{Binding GridVisibility, FallbackValue=Visible}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid Name="SeventhLevelGrid" Grid.Row="0" HorizontalAlignment="Center" Margin="2,15,2,2">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
<ColumnDefinition Width="*" SharedSizeGroup="B"/>
</Grid.ColumnDefinitions>
<TextBlock Grid.Column="0" HorizontalAlignment="Left" Text="Enter:"/>
<TextBox Name="EigthLevelTB" Grid.Column="1"
Text="{Binding UserData, UpdateSourceTrigger=PropertyChanged, ValidatesOnExceptions=True}"
KeyboardNavigation.TabIndex="0">
</TextBox>
</Grid>
</Grid>
</Grid>
</Border>
</Border>
</Grid>
</Grid>
<ScrollViewer Name="FirstLevelScrollViewer" Grid.Column="1" CanContentScroll="True" VerticalScrollBarVisibility="Auto">
<DockPanel Name="SecondLevelPanel" LastChildFill="False">
<GroupBox DockPanel.Dock="Top">
<ContentControl Content="{Binding Legend}" Margin="5" IsTabStop="False" KeyboardNavigation.TabNavigation="None" Focusable="False"/>
</GroupBox>
<GroupBox Name="ThridLevelGB" DockPanel.Dock="Top" Content="{Binding ButtonList}"
Focusable="False" KeyboardNavigation.TabNavigation="Continue"
VerticalAlignment="Top" VerticalContentAlignment="Stretch" >
</GroupBox>
</DockPanel>
</ScrollViewer>
</Grid>
<ContentControl Name="OverlayContent" Content="{Binding OverlayContent}"/>
</Grid>
Thanks,
RDV