The short answer is that he can't. The president has no power to ban TikTok. The president also has no control over how the app is distributed or its connectivity. He can say whatever he wants, and he can strongarm the agencies he has some control over (like the FCC), but there's simply no mechanism by which a ban can be enforced, legal or technical. Unless Google and Apple decide to voluntarily remove it from their app stores and forcibly remove existing downloads, it's not going away, and neither company is going to do that without a legal battle. There's no one who even has the authority to demand it, though.