Catalyzes the transfer of the GlcNAc-1-phosphate moiety from UDP-GlcNAc onto the carrier lipid undecaprenyl phosphate (C55-P), yielding GlcNAc-pyrophosphoryl-undecaprenyl (GlcNAc-PP-C55). di-trans,octa-cis-undecaprenyl phosphate + UDP-N-acetyl-alpha-D-glucosamine = N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UMP Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Belongs to the glycosyltransferase 4 family. WecA subfamily.