Underbarrel Grenade Launcher not working/able to be equipped

I have unlocked the underslung rail attachment which I'm assuming is used for the underbarrel grenade launchers (HE, smoke, incendiary etc). Howeber, how do I actually use it? 

I have the underslung rail attachment equipped but I can't find the grenades that are used for it. Is it bugged? What is the button on PS5 for using the underbarrel grenade launchers. 

None of this is very clear in the game settings. I don't see smoke/HE/incendiary grenades in the weapon customisation screen.