How can i make my imported 3d model collidable?

Dear Community,
For a school project i am making a game. I imported a 3d model of an airport as scenery, but have no idea how to make my character collide with it, because now he is just walking through the walls. I tried a mesh collider and box collider, they didd’nt work, and when i placed a rigidbody my ground texture suddenly dissapeard, Please Help.

Maxxb12

Select your model in the project tab, mark “generate coliders” and click apply.