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