solving #Value error on User defined formula reading from different workbooks

Hi, this is the situation:
I have a macro that creates a number of rows and in some columns there are formulas that call userdefined formulas and external files to get results. When I execute the macro to create the rows there is no problem and even work with solver.

if at some point one of the external files is closed and a formula is edited (with no actual change) the formula returns "#Value!"
to sove that I can reopen the file and by going to edit-->replace "=" by "=" all gets recalculated and correct results are returned. (normal recalculate --F9-- does not work, but I can live with that)

up until here is quite understandable.

HOWEVER:

I recorded a macro to do it exactly programatically and it does not work
any clues on the reason / possible workaround??

Sub Macro13()
    
'
    Cells.Replace What:="=", Replacement:="=", LookAt:=xlPart, SearchOrder _
        :=xlByRows, MatchCase:=False, SearchFormat:=False, ReplaceFormat:=False
End Sub

and this is the user defined function:

Public Function InterpolateFromExperimentalData(ByVal Q As Variant, ByVal t As Variant, fn As String) As Double
    'Q is the horizontal dimension
    'T is the vertical dimension
    Dim owb As Workbook
    Dim f As Worksheet
    Dim lc As Integer
    Dim lr As Integer
    
    
    For Each wb In Application.Workbooks
        If wb.fullname = fn Then Set f = wb.Worksheets("DATA")
    Next
    
    If f Is Nothing Then
        Set owb = Workbooks.Open(fn, readOnly:=True)
        Set f = owb.Worksheets("DATA")
    End If
    
    lc = lastColumn(ws:=f, r:=1)
    lr = LastRow(ws:=f, C:=1)
    
    
    InterpolateFromExperimentalData = Linearinter22d(f.Rows(rgr(1, lr)).Columns(rgc(1, lc)), t, Q)
    
    
End Function

and other formulas contained

Public Function Linearinter22d(inputs As Range, x As Variant, y As Variant) As Double
'adapted from http://www.ozgrid.com/forum/showthread.php?t=32957&p=165188#post165188

    Dim nx As Long, ny As Long
    Dim lowerx As Long
    Dim lowery As Long
    Dim upperx As Long
    Dim uppery As Long
    Dim i As Long
    
    nx = inputs.Rows.Count
    ny = inputs.Columns.Count
    
    If x <= inputs.Cells(2, 1) Then
        lowerx = 2
        upperx = 3
    ElseIf x >= inputs.Cells(nx, 1) Then
        lowerx = nx - 1
        upperx = nx
    Else
        For i = 2 To nx
            If inputs.Cells(i, 1) >= x Then
                upperx = i
                lowerx = i - 1
                Exit For
            End If
        Next
    End If
    
    If y <= inputs.Cells(1, 2) Then
        lowery = 2
        uppery = 3
    ElseIf y >= inputs.Cells(1, ny) Then
        lowery = ny
        uppery = ny - 1
    Else
        For i = 2 To ny
            If inputs.Cells(1, i) >= y Then
                uppery = i
                lowery = i - 1
                Exit For
            End If
        Next
    End If
    
    Dim XL As Double, XU As Double, YL As Double, YU As Double
    Dim temp1 As Double, temp2 As Double
    
    XL = inputs.Cells(lowerx, 1)
    XU = inputs.Cells(upperx, 1)
    YL = inputs.Cells(1, lowery)
    YU = inputs.Cells(1, uppery)
    
    temp1 = (inputs.Cells(lowerx, lowery) * (XU - x) _
        + inputs.Cells(upperx, lowery) * (x - XL)) / (XU - XL)
    
    temp2 = (inputs.Cells(lowerx, uppery) * (XU - x) _
        + inputs.Cells(upperx, uppery) * (x - XL)) / (XU - XL)
    
    Linearinter22d = (temp1 * (YU - y) + temp2 * (y - YL)) / (YU - YL)

End Function

Function rgr(ByVal firstRow As Integer, ByVal LastRow As Integer) As String
    'returns the string to refer to a range of rows as in ws.rows(string)
    rgr = firstRow & ":" & LastRow
End Function

Function rgc(ByVal firstcolumn As Integer, ByVal lastColumn As Integer) As String
    'returns the string to refer to a range of columns as in ws.columns(string)
    rgc = N2L(firstcolumn) & ":" & N2L(lastColumn)
End Function

Public Function N2L(num As Integer) As String
    'returns letter corresponding to column number
    N2L = Split(Cells(1, num).Address, "$")(1)
End Function

you need to make your udfs volatile or pass them all the ranges they require as arguments so that they recalculate automatically. then you shouldn't need to bother replacing = with =