How to Destroy Terrains (destroy(this.gameobject) not working)

Hi guys!

So I currently have a game that's set up to randomly instantiate game objects from an array for a endless runner. I've recently tried to make it so that terrains can be instantiated as I'm not the best 3D modeller in the world.

I've made each terrain a prefab, which is great and allows for the terrains to be generated indefinitely, which is fantastic! But as you pass the terrain and it goes out of the camera, they aren't being destroyed. I usually use OnBecameInvisible to destroy a object, but this really isn't working for the terrains, and I certainly don't want terrains continuing to exist past the camera and hogging all the memory.

Any advice would be greatly appreciated!