That was a huge discussion about that. *.Nextcloud.com - connectivity issues
Short answer: you can always use Github as mirror: Releases · nextcloud/server · GitHub
Long answer:
We kind of “find-out” that currently there is no default feature that’s enables this “pre download” → It will be deleted by default if you do this: *.Nextcloud.com - connectivity issues - #60 by kesselb
So there is a ticket in github and solution for it (but not implemented)
If you fails, you can always use this workaround: