Hi Spiris.
You should not directly get steam online subsystem. Instead of that UE4 provide general interface to working with OnlineSubsystems such like Steam, GameCenter (IOS), GooglePlay, etc. To get this interface try this:
#include "OnlineSubsystem.h"
auto pOnlineSubsystem = IOnlineSubsystem::Get();
To set up achievement progress use this function
pOnlineSubsystem ->GetAchievementsInterface()->WriteAchievements(…);
Hope it helps!