Try Multi-User Editing plugin. It comes with Unreal.
I use everyday with my partner so we can work in the same project at the same time.
Its a bit buggy and you have to create a new fresh session almost everyday but it works.
Its mostly made for local networks but we are using in with VPN (hamachi) without problems.
If you want we can try to help you in the setup proccess…is not too hard.
Good luck!