Put Hetzner Cloud instances in a placement group
https://registry.terraform.io/providers/hetznercloud/hcloud/latest/docs/resources/placement_group
https://docs.hetzner.com/cloud/placement-groups/overview/
Placement groups are used to influence the distribution of interdependent virtual servers in our data centers. The distribution of the different instances within a group is based on a pattern specified in the type. By enforcing certain rules on the placement of instances within our infrastructure, availability can be influenced in a way that fits your needs best.
In spread placement groups, all virtual servers are running on different physical servers. This decreases the probability that some instances might fail together.