A sizeable human population was separated from the rest of the Earth people for a long time, long enough for allopatric speciation to happen. They are no longer Homo sapiens and now have ZERO reproductive compatibility with the ordinary Earth human. At the separation time, this "away team" had access to all the knowledge and technology XXI century humanity have. Environment and population size was not an issue to their survival.
It is still undecided if this "other humanity" was separated in another dimension, planet or by another means, but there were absolutely no transmission of genetic material between Earth and the away team from the time of separation to the moment of this "first contact" in our story.
How long should these two human populations be separated for this speciation to happen?