Unable to fire Private Sub Worksheet_SelectionChange on user Input

Good Day All (Please see Most recent post below)

VBA Skill: Pretty Basic
Excel type: Excel 2010

Problem:
Range("A1:A10") is where my users will enter a text structure; 1.0, 1.1, 1.2, 2.0, 2.1 etc. If the user enters a text string that includes "0" then we would like the code to activate and place a formula Offset(0, 4) from the ActiveCell, the following code is cobbled together from various posts and works to some extent:

Option Explicit


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Not Application.Intersect(Target, Me.Range("A1:A10")) Is Nothing Then
        ' At least one cell of Target is within the range A1:A10.
        ' Carry out some action.
        If InStr(ActiveCell, "0") Then
            ActiveCell.Offset(0, 4).Formula = "=SUM(1+2)"
        Else
        'Cell does not = 0 So Leave Code
        Exit Sub
    
    End If
    End If

End Sub

This code activates if we have entered 1.0, 2.0, 3.0 etc however it only works after we return to the Cell (with Keyboard Navigation Keys), what do we need to do to make the code activate on "Enter" or when we leave the ActiveCell; i.e. User inputs 1.0 hits Enter, Tabs or Navigates using the arrow Keys Then Code Activates.

I wondered if the following would be more effcient than ActiveCell but i can't get it to work with the above code

 If Not Application.Intersect(Target, Me.Range("A1:A10")) Is Nothing Then
        ' At least one cell of Target is within the range A1:A10. 
        ' Carry out some action.

Any help will be gratefully received

As advised below; Apologies for delay, my work Internet is rubbish!
Test Macro Entry via VBA.xlsm