Make sure that the server can access the backups, for example by copying the backups to the server, and restore the VMs. What is the problem?
EDIT: There is also the new command-line qm remote-migration, if both nodes/clusters can reach each other (temporarily) via the internet.
EDIT2: If you are worried about (privately) connecting the two systems, look at wireguard to setup a private tunnel (or just use plain ssh, scp, sshfs).