In NodeBB, subcategories are referred to as child categories. In order to set up your subforum, you would create your necessary categories (one for mobile phones, and then others for android, ios, etc) and then set the parent category of your intended subforums to be your mobile phones category.
I don't have an answer for you...
However, I did note that you are using PG. To that end, make sure to update the driver: