Catalyzes the hydroxylation of 2-nonaprenyl-3-methyl-6-methoxy-1,4-benzoquinol during ubiquinone biosynthesis. a 6-methoxy-3-methyl-2-all-trans-polyprenyl-1,4-benzoquinol + AH2 + O2 = A + a 3-demethylubiquinol + H2O Binds 2 iron ions per subunit. Cofactor biosynthesis; ubiquinone biosynthesis. Belongs to the COQ7 family.