The report's sorting and grouping options control the sort sequence of a report NOT the order by clause of the query. If the field that you want to sort on is NOT in the report's recordsource, there is NO way to sort on it.
If you want the report sorted by some column in the subreport's recordsource, you're going to have to invert the relationship. Make the subreport the main report and make the main report the subreport.