I would suggest looking at Syncthing. It’s not perfect by any stretch, but it works peer to peer, without any kind of central host, ip or domain name requirements. You simply install it on the client machines, and they work out how to talk to each other over any available networks.

Beware changing the casing on your files or directories though, Syncthing was made entirely case sensitive, which does not play nice with Windows.

One very nice feature is that it does have an android client (f-droid.org/…/com.github.catfriend1.syncthingandr…), and it supports full background syncing to your local storage on the phone. Great for syncing your photos, but also music. You add some mp3’s on your desktop computer, and by the time you’ve put on your jacket they’re on your phone ready to listen to without any ‘service’ getting in your way.