Female led relationships discussion

I’m a KH/Domme managing my sub who lives overseas. I need more ideas on how to keep control over him. Right now, communication itself is tough; even though he wants my control just as much as I do, the distance makes it feel nearly impossible. So without dependable internet or phone service, what options are there? Also, once those connection issues improve, I’m looking for ways to maintain authority from afar and would welcome suggestions! Thanks in advance!