C# WPF combobox in datagrid to have a different source to the datagrid

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C#

WPF

combobox

DataGrid

Binding

I am writing a C# WPF application and following the MVVM pattern.
My ViewModel has an object in it called NetworkMain
My View has a DataGrid which binds to an ObservableCollection called FlowControls which is inside NetworkMain

My DataGrid Itemsource is defined as follows:

<DataGrid ItemsSource="{Binding Path=NetworkMain.FlowControls}">

The above is working fine.

I want one of my columns in the DataGrid to be a ComboBox.
My column is defined as shown in the code at the bottom of this question.

The problem I have is I want the source of the ComboBox to use a collection called Pipes which is in NetworkMain and not FlowControls.

How can I do this?
I tried using Binding Path=NetworkMain.Pipes but I think it is looking for NetworkMain from within FlowControls which isn't correct.

Hope the above makes sense.

What I have tried:

<DataGridTemplateColumn Header="Downstream Pipe ID">
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <ComboBox 
                SelectedItem="{Binding Path=SelectedDownstreamPipeID, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                ItemsSource="{Binding Path=NetworkMain.Pipes}"
                DisplayMemberPath="Description" >
            </ComboBox>
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>

EDIT

I have updated the XAML as follows and I now get the list I want in the ComboBox. What I can't get to work is the setting of the selected item.
I get the following error:
System.Windows.Data Error: 1 : Cannot create default converter to perform 'two-way' conversions between types 'Drain.Models.Network.Conduit' and 'System.String'. Consider using Converter property of Binding.

<UserControl.Resources>
    <CollectionViewSource 
        x:Key="Conduits" Source="{Binding NetworkMain.Conduits}">
    </CollectionViewSource>
</UserControl.Resources>

<DataGridComboBoxColumn 
    Header="Downstream Pipe ID"  
    SelectedItemBinding="{Binding outgoingConduit, Mode=OneWayToSource}" 
    ItemsSource="{Binding Source={StaticResource Conduits}}" 
    DisplayMemberPath="ID"/>

public ObservableCollection<Conduit> Conduits { get; set; } = new();

public Conduit outgoingConduit 
        { 
            get; 
            set; 
        }

I tried adding the following convertor:

<conv:ConduitObjectConvertor x:Key="ConduitObjectConvertor" />

SelectedItemBinding="{Binding outgoingConduit, Mode=TwoWay, Converter={StaticResource ConduitObjectConvertor}}"

I copied and pasted a convertor from an example online and I thought I adapted it correctly but I still get errors. The convertor is here:

public class ConduitObjectConvertor : TypeConverter
    {
        public override bool CanConvertFrom(ITypeDescriptorContext context,
        Type sourceType)
        {
            return sourceType == typeof(Conduit);
        }

        public override object ConvertFrom(ITypeDescriptorContext context,
            System.Globalization.CultureInfo culture, object value)
        {
            return value as Conduit;
        }

        public override bool CanConvertTo(ITypeDescriptorContext context,
            Type destinationType)
        {
            return destinationType == typeof(Conduit);
        }

        public override object ConvertTo(ITypeDescriptorContext context,
            System.Globalization.CultureInfo culture, object value, Type destinationType)
        {
            return value == null ? null : value as Conduit;
        }
    }

The error I get is:

InvalidCastException: Unable to cast object of type 'Drain.Helper.Convertors.ConduitObjectConvertor' to type 'System.Windows.Data.IValueConverter'.