Probably obvious to most but for those like me who backed their head on it a bit before the light dawned, if your Nextcloud instance is internal to the firewall you're backing up it won't work unless you add a DNS override for the URL of your Nextcloud. In this case the internal Nextcloud was also behind reverse proxy so the override was URL -> proxy IP, but I'm assuming a direct to NC internal IP would also work where it's a direct port forward.
You can use the IP address with HTTP and no virtual hosts on the server however if you use HTTPS, you need a certificate that can be verified and that usually involves a hostname.