Where are you looking?
On the (iOS?) app?
Or on the website?
On the website it's here:
On the app it's the Buy More button beside your internet allowance balance.
As you seem to understand buying a new membership does not start that membership and give you access to the membership allowances until the current membership has run its full monthly course and expires at which point the new membership which has stacked will kick in.
Hope this helps.
Glad that you got sorted.
The link that I posted should show you where the add-on purchase page is on the website.