No one seems to know about it, but there is now a 3rd party app: https://apps.apple.com/us/app/m%C3%B6bius-sync/id1539203216
The developer made it with a little help from the syncthing team. There are comments there: https://forum.syncthing.net/t/mobius-sync-ios-client-now-in-...
I've been using it for some months, and it works fine, when it works. I sometimes have to start it manually because iOS killed it in the background.