Adding physics to imported fbx asset??

I’m trying to add physics to an imported fbx asset so that I can walk around on it, it’s a ground plane.

From what I can work out I should simply be able to check the ‘generate colliders’ button under fbximporter, click ‘apply’ and that’s it. However it doesn’t work for me as when I play the game I just fall through the world?

Does anyone know what I’m doing wrong?

Any help greatly appreciated.

What are you using to walk around with? First Person Controller Prefab?

Is it above the plane before the game starts?