While Ren was gathering allies for the final fight, Tsukiyomi who had followed after Ren once their children had grown big enough to take care of themselves. She unlike Ren was already powerful enough to reach the higher realms from the very beginning.
So it wasn't that hard for her to go to the upper realms to go and look for Ren, unfortunately, the upper realms were basically the size of many of the universes gathered together. It was basically the place where most of the Gods fled to during the war with the original incarnation.
It used to be a place that no mortal being except for the Strongest Mortal Soul could enter. It was a place filled with powerful Gods and Dragons, but nowadays there were many so-called mortals living in the upper realms. It was not because all of them were powerful enough, but because the truly powerful lived in the upper realms and gave birth in the upper realms.
