If disaster occurs and it's necessary to remotely access the machine, boot into NixOS install media and access the machine using SSH.
By default it's possible to SSH into the install media as root. In order to log in create a password.
Start SSH Daemon.
Check the IP address.
Now it should be possible to access the machine as root.
NOTE: SSH'ing into a machine as root is a terrible practice and is only done here temporarily.