The packages are available, but it might have been the MyGet service which was down at the time you did the restore. We added Feedz.io as an alternate source due to ongoing issues with MyGet, as well as GitHub Packages but that requires authentication with GitHub to use it.
If you don’t need the pre-release packages, best to just use NuGet as you did, otherwise we have documentation on the various options above you can follow to try another source.