I was able to log in to my server. First thing I was prompted to do was update my server password.

Next I generated an SSH key:

$ ssh-keygen -t ed25519 -C "<email@domain.com>" -f ~/.ssh/id_hetzner

Since I had created the server already, I had to add the contents of the .pub file to the root server:

$ echo "keyfile_content" >> /root/.ssh/authorized_keys

Now I can SSH into Mojave.