How to calculate formula with data in file opened with GetOpenFilename?

I wrote code to reformat a workbook by separating and combining information on separate sheets and then save every sheet separately as a CSV.

The beginning of my code:

Sub All()
Dim Bottom As Long
Dim Header As Long

> 'A. CHECK DATE

If ThisWorkbook.Sheets("ACH PULL").Range("C1") <> Date Then
MsgBox "ERROR" & Chr(10) & "Date on file is different than today's date" & Chr(13) & "Ask client for corrected file"
Exit Sub
Else

> '1. OUTGOING CHECKS

Sheets("OUTGOING CHECKS").Select
Bottom = WorksheetFunction.Match((Cells(Rows.Count, 1).End(xlUp)), Range("A:A"), 0)
Header = WorksheetFunction.Match("Account*", Range("A:A"), 0)
If Bottom <> Header Then
MsgBox "ERROR" & Chr(10) & "The batch contains outgoing checks" & Chr(13) & "Ask client for corrected file"
Exit Sub

Bottom and Header are used to find the header of the range and the last row respectively. I use this so many times in my code on separate sheets.

The code works when I run it from the file that I need to modify. But I need to assign it to a button to another spreadsheet to open the to-be-modified file through VBA and then apply the code. So I added this:

Sub All()
    Dim FileToOpen As Variant
    Dim NewBatch As Workbook
    Dim Bottom As Integer
    Dim Header As Integer
    
    FileToOpen = Application.GetOpenFilename(Title:="Find batch file")
    If FileToOpen <> False Then
        Set NewBatch = Application.Workbooks.Open(FileToOpen)
    End If
    
    'A. CHECK DATE
    
    If Sheets("ACH PULL").Range("C1") <> Date Then
        MsgBox "ERROR" & Chr(10) & "Date on file is different than today's date" & Chr(13) & "Ask client for corrected file"
        Exit Sub
    Else
    
        '1. OUTGOING CHECKS
    
        Sheets("OUTGOING CHECKS").Select
    
        Bottom = WorksheetFunction.Match((Cells(Rows.Count, 1).End(xlUp)), Range("A:A"), 0)
        Header = WorksheetFunction.Match("Account*", Range("A:A"), 0)
    End If

    If Bottom <> Header Then
        MsgBox "ERROR" & Chr(10) & "The batch contains outgoing checks" & Chr(13) & "Ask client for corrected file"
        Exit Sub
        ' .. The rest of the code

At the line:

Bottom = WorksheetFunction.Match((Cells(Rows.Count, 1).End(xlUp)), Range("A:A"), 0)

I either get 1004 or 400 error.

I have the two pieces (opening a workbook, and reformatting) working separately, but I can't combine them.

I Dim'd the two integers that I need to use before using them. I tried making multiple changes including NewBatch.Activate.

It didn't made a difference as the opened workbook is already activated. I tried to set the values for Bottom and Header.

Something like this maybe:

Sub All()
    
    Dim FileToOpen As Variant
    Dim NewBatch As Workbook
    Dim Bottom As Long, Header As Variant 'not Long
    
    FileToOpen = Application.GetOpenFilename(Title:="Find batch file")
    If FileToOpen = False Then Exit Sub 'user cancelled open
    
    Set NewBatch = Application.Workbooks.Open(FileToOpen)
    
    'A. CHECK DATE
    If NewBatch.Sheets("ACH PULL").Range("C1").Value <> Date Then
        ProblemMsg "Date on file is different than today's date." & _
                    vbLf & "Ask client for corrected file"
        Exit Sub
    End If
    
    '1. OUTGOING CHECKS
    With NewBatch.Sheets("OUTGOING CHECKS")
        Bottom = .Cells(.Rows.Count, 1).End(xlUp).Row 'last entry in Col A
        Header = Application.Match("Account*", .Range("A:A"), 0) 'not WorksheetFunction.Match

        If IsError(Header) Then 'make sure we located "Account*"
            ProblemMsg "'Account*' not found in ColA on sheet '" & .Name & "'"
        Else
            If Bottom <> Header Then
                ProblemMsg "The batch contains outgoing checks." & vbLf & _
                           "Ask client for corrected file."
                Exit Sub
            End If
        End If
    End With
    
    '...
    '...
End Sub

'Utility sub for displaying error messages
Sub ProblemMsg(msg As String)
    MsgBox "ERROR" & vbLf & msg, vbExclamation, "Please review"
End Sub