Can you tap into the chimney or another existing vent? If so, read up on local code as there is a right way and wrong way to do these kind of mods. You want to do it the right way... I'm not all together clear on your setup so it is hard to give adequate advice. It seems you have a grow tent in a closet venting from and to the attic? Is that correct? Can you give us more info on your fan? What is the make? Is it inline, squirrel cage, or other? What is the CFM rating or is that the same as m3/h? I'm thinking it is. Also, do you have a fan controller so you can dial in the speed to suit your needs?

It sounds to me like your fan is under powered or that you are drawing hot air from the attic which isn't able to adequately cool your tent/light.