Well, it is obvious that Trump cannot come up with the money to build his promised wall, nor is it even feasible to build it across large parts of the border with Mexico, as anyone who has been to, say, Big Bend National Park will know. So, what to do, to avoid another gigantic humiliation?
Here is what I think will happen: Trump will find enough money somewhere to start building the wall, and then actually build a few miles or a few dozen miles of it, which will be utterly useless, except for propaganda purposes. The completion of the wall will then fall into the same hole as Obamacare repeal; always promised, but never happening. His deluded followers will take his meaningless gesture as having fulfilled his promise, and the rest of us will just move on to his next abomination. That's how he wins until we are sick of winning.