Ramanujacharya did not write commentaries on Upanishads. He only wrote commentaries on Gita and Brahmasutra. Then there must be somebody in Vishistadvaita tradition who did the job. Who is the person?
Ranga Ramanuja muni has written commentaries on the principal upanishads based on the Vishistadvaita tradition. His work has been translated by Dr. N. S. Anantha Rangachar and available here Vedanta Desikan has also written a commentary on the Isha upanishad.