It is kind of strange for a superstructure like the TET to go offline when it goes beyond the horizon. Why is it that it hasn't satellites? We are far less technological than them and we use satellites nowadays to see football live.
If the Tet were always online and communicating, Jack and Vika would inevitably begin to wonder why Sally was the only human they ever spoke with and how Sally managed to be awake and sane 24 hours a day. The real reason of course is that Tet only knew how to imitate Sally, so the Tet's cover story had to account for that. Thus Jack and Vika are told that they are part of a small "mop-up crew" with the rest of humanity on Titan and inaccessible. Sally and the crew of the Tet "go offline" on a diurnal cycle so that they can recreate, sleep and eat, which gives Jack and Vika time to do the same. It passes credibility that Jack and Vika would operate this way for five years without question, but an always cheerful, never sleeping Sally would give the game away much faster.