Mannosyltransferase that operates in the biosynthetic pathway of dolichol-linked oligosaccharides, the glycan precursors employed in protein asparagine (N)-glycosylation. The assembly of dolichol-linked oligosaccharides begins on the cytosolic side of the endoplasmic reticulum membrane and finishes in its lumen. The sequential addition of sugars to dolichol pyrophosphate produces dolichol-linked oligosaccharides containing fourteen sugars, including two GlcNAcs, nine mannoses and three glucoses. Once assembled, the oligosaccharide is transferred from the lipid to nascent proteins by oligosaccharyltransferases. In the lumen of the endoplasmic reticulum, catalyzes the addition of the seventh and ninth alpha-1,2-linked mannose residues to Man(6)GlcNAc(2)-PP-dolichol and Man(8)GlcNAc(2)-PP- dolichol respectively. {ECO:0000269|PubMed:15148656, ECO:0000269|PubMed:15945070}. This is the function of ALG9 (ALG9 alpha-1,2-mannosyltransferase, ENSG00000086848).