I am still learning DataTemplate, but at this moment I would like just to underline (as a note to myself) that it seems that binding for DataTemplate goes to another class.

XAML:

<Window x:Class="ItemsControlItemTemplate.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:itemsControlItemTemplate="clr-namespace:ItemsControlItemTemplate"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>

      <Grid.Resources>
        <itemsControlItemTemplate:ItemsControlViewModel x:Key="ItemsControlViewModel" />
      </Grid.Resources>
      
      <ItemsControl DataContext="{StaticResource ItemsControlViewModel}" ItemsSource="{Binding myItemsSource}">
        <ItemsControl.ItemTemplate>
          <DataTemplate>
            
              <DataTemplate.Resources>
                <itemsControlItemTemplate:AnotherModel x:Key="AnotherModel" />
              </DataTemplate.Resources>
            
              <Label DataContext="{StaticResource AnotherModel}" Content="{Binding myContent}" />
            
          </DataTemplate>
        </ItemsControl.ItemTemplate>
      </ItemsControl>
      
    </Grid>
</Window>

Here note part:

<DataTemplate.Resources>
	<itemsControlItemTemplate:AnotherModel x:Key="AnotherModel" />
</DataTemplate.Resources>

<Label DataContext="{StaticResource AnotherModel}" Content="{Binding myContent}" />

My view model:

public class ItemsControlViewModel
{
	public IEnumerable<string> myItemsSource { get; set; }

	public ItemsControlViewModel()
	{
		myItemsSource = new[] { "Test1", "Test2" };
	}
}

public class AnotherModel
{
	public string myContent { get; set; }

	public AnotherModel()
	{
		myContent = "test";
	}
}

Here notice class:

public class AnotherModel
{
	public string myContent { get; set; }

	public AnotherModel()
	{
		myContent = "test";
	}
}

Which is actually in use in the label under the datatemplate

Example project you can download from here.