Numbered headings not working as expected after customising headings

[seanspotatobusiness]

I customised the headings to make the relationship between them clearer (some of the defaults were hard to tell which was a heading which was a subheading).

I now want the headings to be numbered in the relationship:

1. Heading
1.1 Subheading


1.1.1 Subsubheading

When I follow the instructions here: https://support.office.com/en-gb/art...ad=GB&fromAR=1

it affects only the first heading and none of those below it:



Does anyone know how to fix this? Thanks.