How Do I Use a Custom List for my Slicer

I am new to using charts and I have run into a problem with sorting the options on a slicer.

I want to sort the slicer by month but with the year commencing in October.

I understand that I need to create a custom list and I have done that and saved in the advanced Options. but the formatting stubbornly refuses to let me sort using this list.

What have I done wrong? and how do I rectify it.

Because I have taken the worksheet out of workbook, not all of the data is reflective of the true file, but the slicer does seem to be as in the main book.

I did read that I should be able to press the refresh button, but it is greyed out seemingly permanently.