I would verify that the new hub/C-8 has Internet connectivity. If you've already set the hub up, one way to test is this: go to One way to test: go to Settings > Network Setup, then launch the "Network Test" tool from the link at the upper right. Use it to try to ping cloud.hubitat.com (or really anything on the Internet, though this one probably makes the most sense). If you've configured a static IP address on the hub, I'd make sure your DNS servers are correct, or just switch it back to DHCP at least temporarily to see if that helps. If you've configured a DHCP reservation on your home router, make sure it doesn't conflict with an IP address in use by another device (many won't let you set this up in the first place, though it could still happen for various reasons).
While you're there, check Settings > Hub Details to see if you really are on the latest version and use Settings > Check for Updates if you're not. If your hub couldn't download the cloud backup and Internet connectivity was the problem, it's possible it couldn't have updated itself automatically during the onboarding process like it's supposed to have. But this won't work until the network issue is resolved, again assuming that's the problem.