New workbook template not working (365)

I have a simple template I want to use for all new blank workbooks. I'm led to believe I should save it as "book.xltx" in "the XLSTART folder". Well I have two XLSTART folders:

C:\Users\me\AppData\Roaming\Microsoft\Excel\XLSTART\

C:\Program Files\Microsoft Office\Root\Office16\XLSTART\

I have saved it in both locations, but new books still open without the template.

I have also tried defining my own location in "Options > Advanced > General > Open all files in" and saving it there, but no luck there either.


What am I doing wrong?!

Save this file at this location

C:\Program Files\Microsoft Office\Root\Office16\XLSTART\

and Save the excel workbook as "book.xla" (Microsoft Excel Add ins) in "the XLSTART folder"