It is no client <-> server principal.
All machines that running syncthing are a note.
You have to add on all notes all other notes to sync with them, see:
https://forum.syncthing.net/t/getting-started/46?source_topic_id=431Maybe to ask: Why is the setup so complicated? Read:
https://forum.syncthing.net/t/frequently-asked-questions-faq/405?source_topic_id=431