Just received from Hetzner support
Thank you for your request.
I’m sorry to inform you that CAX servers only support UEFI boot.
If you need any further assistance, please feel free to contact us again.
Kind regards
Dad B*w
Hetzner Online GmbH
try to install with
curl -L https://download.mikrotik.com/routeros/7.17.2/chr-7.17.2.img.zip | funzip | dd of=/dev/sda bs=1M
On Hetzner Ampere CPU vps CAX21