Use Syncthing: open source, free, encrypted & secure, and decentralised (i.e. direct peer-to-peer synchronisation without a central cloud server).
And quite easy, at that. You just select which vault folder you want to share, add a trusted device (the other device that should also keep a copy of the vault), set up the new device, and let it sync. After it’s done, point your Obsidian mobile to the newly-created vault folder locally. Voila!