You see, the backup file is usually several G.
Too big to download, maybe need 10hours.
So it should support download resume from Breakpoint.
I have downloaded my backup file 2 times, but the network is not so steady and failed either these 2 times.
You could download the backup via SFTP from your server. It allows resume. If you are on Windows, a client like WinSCP will allow you to resume the download. Backups are usually stored in /var/discourse/shared/standalone/backups/default