Install the server with:
nix run github:nix-community/nixos-anywhere -- --generate-hardware-config nixos-generate-config ./hardware-configuration.nix --flake .#onemore --target-host root@<ip>
Then, connect to the server with ssh server@<ip> and set the password using the passwd command.
To push changes run:
nixos-rebuild switch --flake .#onemore --ask-sudo-password --target-host server@<ip>
In case the modpack is updated, run nix flake update modpack to update the input. Changing the packHash will be required.