That's probably a bit of an exaggeration.
A POTUS does have certain powers afforded to him by the constitution and subsequent legislation that doesn't require congressional authority. EX: he is the Commander in Chief, executive orders, Recess appointments, War Powers Acts, and certain emergency powers.
And he has other powers that simply require the advice of congress. EX: Treaties, tariffs, etc.
And he has Veto power. Consequently, Congress can override a Presidential Veto by a 2/3s vote in each chamber.
I will say again that I don't think Trump's actions in this matter are unconstitutional, and Congress still has a means of stopping him by a Veto override. If this really is a critical mistake, Congress would be doing just that. Whether or not they are wise or best for the country is certainly questionable and up for debate.
Lest we forget that there is yet another check and balance that could stop this as well; the SCOTUS. Points being...The system is still working as it applies to this issue. What Trump has done thus far, whether wise or not, is a workaround that he is authorized to use beginning with certain wording in the Constitution and later reinforced and expanded by Congress. And the SCOTUS has previously upheld it.
https://en.wikipedia.org/wiki/National_Emergencies_Act
I think most or all of us here will agree that the wall needs to be built. The question of why the GOP didn't fund it when they had the power is absolutely a valid one, and I, for one, would like an answer to that. Most of us will probably agree that the way this is being handled is definitely less than ideal, and potentially troublesome.
But is it a Constitutional Crises? Absolutely not! Trump is playing the system, and I believe without the boundaries of the Constitution and subsequent law and court rulings. There are at least two Constitutional means of stopping it, if it really is so terribly wrong.
One final opinion from me...though I certainly wish this would have been handled differently, it is way down the list of things our government is doing that worry me. The insanity coming from the far left and the apparent (to me, anyway) fact that entire portions of our government is corrupt and compromised is a far greater concern to me. And until We The People stop being sheeple who are hearded by partisan politics and a corrupt media, I'm afraid it's not likely that this will get any better. BTW, my previous sentence is not aimed at anyone in particular (at least here); just a general statement. Our electorate is horribly ignorant of the facts, and certain people and powers are actively working to keep them that way.