The installation kubeadm-dind-cluster requires Internet, because both the script itself and kubeadm inside pulls Docker images from Docker Hub and Google Container Registry.
One way to avoid the Internet requirement is to host a local Docker registry preloaded with all images required and configure the cluster to pull from this local registry.
The setup below demonstration air gap installation of Kubernetes v1.13, tested with Ubuntu 18.04 and Docker CE 18.09.6.