Step out of the cab and walk to the cars you want to uncouple manually, or use the 8 or 3 camera to fly to where you want to uncouple and do it that way. You do so by clicking the appropriate parts, I don't think there's any keybinds specifically for (un)coupling.
Not sure if this applies to you but the on-foot camera sometimes makes these click spots hard to get to, in which case you can try to crouch, or if the wagon has a small platform, you can try to climb that. Usually, the fly camera makes coupling easy.
In which case, do what the others say. While walking, you can press C to crouch. This will make it easier to aim past the buffers.