I just found these posts after losing a Rancher installation. Clusters are still up, but no obvious way to import them into a new Rancher. Still got SSH access to the nodes?
Go to your new Rancher, choose “Add cluster”, “Import existing cluster”. After entering a name and pressing “Create”, it will print the commands for importing the existing cluster.
Now, SSH into the head node and check what containers are running (we use Docker, so Docker example below):
# Login with SSH
Last login: Fri Feb 7 16:41:52 2020 from xxx.xxx.xxx.xxx
# Find container running the cattle node agent
docker ps | grep k8s_agent_cattle-node-agent
ff6141bbe174 87468cfad9b5 "run.sh" About a minute ago Up About a minute k8s_agent_cattle-node-agent-vklg9_cattle-system_...
# Start a shell within this container
docker exec -it ff6141bbe174 /bin/sh
# Execute the join commands
sh-4.4# curl --insecure -sfL https://<rancher ip here>/v3/import/<unique token from rancher, see import command>.yaml | kubectl apply -f -
Too late for OP, but hopefully someone can benefit from this.