Before adding a machine to a cluster, you need to be sure that the rescue system can be reached reliably.
You can ask Hetzner support to do that. Use this text when contacting support:
Alternatively, you can order a Remote Console and change the corresponding settings in the BIOS yourself.
Common keys to enter the BIOS, depending on the manufacturer:
Press the corresponding key repeatedly while the machine reboots. You can trigger the reboot via the Robot Web UI of Hetzner.
The network card with a higher boot priority than the disks:
Almost every BIOS is different, so that we can't provide a detailed instruction how to change the boot order. Your favorite search engine will likely tell you how to configure the BIOS of your machine.