The record sorting order is based on the Airtable view selected in the view's configuration. You can find the option by going to:
Portal Tables
Select a table
Views section
Select an appropriate Airtable View to apply its filter and sort settings.
When no Airtable view is set, the view will be marked as "random order" in the views list, since there will be no sorting applied.
Manual sorting options are also available for the portal user, if the appropriate toggle is enabled in the User Actions section.