Try under services / manage / international roaming. Often the tabs, links and buttons in the Optus account portal are just thrown on a page with no relevance to any section.
There is nothing under Services in my account but you have helped me to find the solution. Clicking 'view details' then 'manage' revealed the roaming option. Thanks very much
Can you please let us know the Optus website that directs to go to the Settings tab so we can get this updated?