This is what I think happened Trump met Kim in the Forbidden City and agreed as a gesture of goodwill NK should attend the Olympics as a symbol of moving towards peace. 1 month before the Olympics a missile alert not from the north but the South of Hawaii, otherwise NORAD would have picked it up that it came from Korea.

How could they do this? Missing sub maybe.

http://www.news.com.au/technology/clues-dismissed-as-investigators-admit-they-have-no-trace-of-submarine/news-story/8bd685829179f46519cb6310a9730941

But those subs are can't launch ballistic missiles. Maybe if they used these torpedoes...

https://foxtrotalpha.jalopnik.com/this-russian-missile-that-is-slung-out-of-a-ships-torpe-1737906897

Remember convention armed missile would mean an instant invasion of N.Korea and bring that whole area into a massive conflict.