BitTorrent Sync is an application based around the peer-to-peer protocol. You can use it to share files between friends and family or simply use it as an efficient way to transfer files within a LAN.
In this article I will briefly go over the steps I went through to Sync my camera photos and WhatsApp media folders from my Android smartphone to my Synology DS215j NAS.
The below guide should also help to get it working on an iOS device. I believe iOS also supports auto syncing just as the Android app does (i.e., you don’t need to have the app open for it to sync).