On iPad I use Möbius Sync, works great. Probably works for iPhone, cannot say.
It’s not free, though, but if you really need to use Syncthing the one-time payment is fine in my opinion. Besides syncing your vault you will be able to sync any other folders as well.
The real problem with iOS/iPadOS is its sandboxing. What I mean by this is that you cannot open the Möbius Sync folder with Obsidian directly and have it work that way. You’ll need to first sync with Möbius Sync, and then copy/paste the files from Mobius folder to Obsidian vault folder. If that’s not too big of an issue for you, go for it. I still haven’t found a way to rectify this i.e. there’s no way to make symbolic links on iPadOS without jailbreaking. 
Oh, and BE CAREFUL!! You need to only copy/paste from Möbius folder to Obsidian vault folder. Don’t cut, delete or modify files in Möbius folder because these changes will reflect in your other synced devices!!!