After installing Unity's new EventSystem, my UI broke. It turns out I had to click the "Replace with InputSystemUIInputModule" in each scene's EventSystem.

EventSystem using the older input system

But I realized that only after deleting the EventSystems.

The new Input System comes with samples
EventSystem using the new input system in some sample project

To have a default EventSystem remade, I uninstalled and reinstalled TMPro. After reinstalling, I added a TMPro button, and it prompted me to import TMPro. I did. At that point it added an EventSystem to my scene, but it was still the old one. Luckily, I had the option to press the button to update it to the new system. Then I selected "Unified Pointer", hoping that would make it work on Mobile as well.

My EventSystem

I tried on my computer, and it worked. Tried it on my Android phone....... And it worked too!!!