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.
StarHeartCoin, a videogame forum
TBH I probably don't have the time to actually work on the forum itself (content). If somebody's interested in becoming an administrator (only qualification = must love videogames?) let me know.
I put it up more so for a demo of Pewter, and a place to showcase a couple of plugins that we wouldn't ordinarily put up on this community.
Big fan of games and am a hardcore PC gamer here. If you need some help getting that site going, I'd be happy to help.