Follow the instructions at Syncthing questions.
As a minor point: Syncthing, Joplin, and iPhones don't work well as a good combination. While it's easy to get Joplin and iPhones to work together (great mobile app, OneDrive/DropBox integration, etc.) and get iPhones and Syncthing to work together (with MobiusSync app), it's impossible to have Joplin use your Syncthing directory on the iPhone. See my investigation on this recently: Synchronize Joplin using local directory on iPhone