Advanced Filter - Range Object Not Accepted (VBA)

I've got a spreadsheet where I run a macro to Advanced Filter results from a "Register" sheet to a "Search" sheet. Macro is run from the Search sheet. Existing code looks like:

Code:
Sheets("Register").Range("A9:Z10000").AdvancedFilter _
Action:=xlFilterCopy, _
CriteriaRange:=Sheets("Register").Range("A3:W4"), _
CopyToRange:=Sheets("Search").Range("A26:X26"), _
Unique:=False

At the moment the macro filters the range down to row 10,000. I need to adapt it to a variable number of rows in case they go over 10,000. Currently I am using a Do loop to find the bottom of the list by having it check the one column out of 26 that will always have data in it (Column D)

Code:
rfRegisterStarts = 10
rfRegisterRowCounter = 0
Do While IsEmpty(Sheets("Register").Cells(rfRegisterStarts + rfRegisterRowCounter, 4)) = False
rfRegisterRowCounter = rfRegisterRowCounter + 1
Loop

Therefore the range of my table to be filtered including header row is:

Range(Cells(rfRegisterStarts - 1, 1), Cells(rfRegisterStarts + rfRegisterRowCounter, 27))

My problem is that trying to change the Range object in front of .AdvancedFilter causes all sorts of errors. I've tried:

Code:
Sheets("Register").Range(Cells(rfRegisterStarts - 1, 1), Cells(rfRegisterStarts + rfRegisterRowCounter, 27))
.AdvancedFilter _
Action:=xlFilterCopy, _
CriteriaRange:=Sheets("Register").Range("A3:W4"), _
CopyToRange:=Sheets("Search").Range("A26:X26"), _
Unique:=False

OR

Code:
Dim rfRegSelect As Range
Set rfRegSelect = Range(Cells(rfRegisterStarts - 1, 1), Cells(rfRegisterStarts + rfRegisterRowCounter, 27))
rfRegSelect.AdvancedFilter _
Action:=xlFilterCopy, _
CriteriaRange:=Sheets("Register").Range("A3:W4"), _
CopyToRange:=Sheets("Search").Range("A26:X26"), _
Unique:=False
With almost every combination of
With Worksheet("Register") or Sheets("Register") I can imagine

Why I'm getting these errors? Is Advanced Filter just really finicky?