That's the clue. How can I get off the Moebius Strip and remain in the present?
A cube won't do it. There we have a mere journey along the single surface. We have a line, we have a plane, we have a cube. And then we have a supercube—a tesseract. That is the logical progression of mathematics. The Bright Ones must have pursued that line of reasoning.
Now I've got to do the same, but without the advantage of being a Bright. Still, it's not the same as expecting a normally intelligent person to produce a work of genius. (Genius by our standards, of course, which I suppose Robert and Star would classify as Tween.) Anyone with a pretty fair I.Q. and proper education and training can follow a genius's logic, provided the steps are there and especially if it has a practical application. What he can't do is initiate and complete that structure of logic. I don't have to, either—that was done for me by a pair of Brights and I "simply" have to apply their findings.
Now let's see if I can.
By reducing the present-past-future of man to a Moebius Strip, we have sheared away a dimension. It is a two-dimensional strip, because it has no depth. (Naturally, it would be impossible for a Moebius Strip to have depth; it has only one surface.)
Reducing it to two dimensions makes it possible to travel anywhere you want to go on it via the third dimension. And you're in the third dimension when you enfold yourself in the twisted cube.
Let's go a step higher, into one more dimension. In short, the tesseract. To get the equivalent of a Moebius Strip with depth, you have to go into the fourth dimension, which, it seems to me, is the only way the Bright Ones could get off this closed cycle of past-present-future-past. They must have reasoned that one more notch up the dimensions was all they needed. It is equally obvious that Star and Robert have followed the same line of reasoning; they wouldn't break their promise not to leave the present—and getting off the Moebius Strip into another present would, in a sort of devious way, be keeping that promise.