WordPress doesn't use the server's cron system. It requires extra configuration on both WordPress and the server to use the server's cron system.
What it does is when a user visits the website, it sees if any schedules need starting, and after sending the user the page they wanted, it then makes a request back to itself to run cron stuff. This sometimes doesn't work if the WordPress can't connect to itself. This is where ALTERNATE_WP_CRON comes in. Instead of making the request itself, when a user visits it forwards the user with a redirect to the page they actually want, and then performs jobs. So the user initiates the jobs, not WordPress itself.
Does the backup eventually start? As in, if you look in the Activity Log, do you see alot of Time Out (which means it started, but had to wait for another cron to start, and it didn't happen in a reasonable time so it aborted) or do you just not see ANYTHING since the one in July?