Great tutorial, helped me figure out achievements 
Quick addition, as mentioned in one of the responses on here: Reset Steam achievements - Platform & Builds - Epic Developer Community Forums
online.ResetAchievements works if you first call ‘CacheAchievements’ blueprint node.
When I tested it in standalone mode I did have to shut down the play session after that, and the next time my achievements would have been reset.
Hope that helps for others to!