VPN is the only way.
Discussion
Sorry, half reply. You can add a local IP (192.168.0.x, etc) to your relay list while we don't have private relay lists. In that way, people know you have a local relay but there is no way to know where.
Would you need to be on your local network to backup then tho? Can you connect to this local relay from the internet?
Correct. If you make something outside of a local network, you have to place it inside a VPN for now. Otherwise, everyone will know you have access to it.
OR, you can use NIP-42 to authenticate with your public keys to write and read from the relay.
You can also use something like tailscale or wireguard