It sounds like -- at some point in the past -- you allowed these nodes to form independent clusters. You cannot merge clusters together. The simplest fix is to wipe the data path on the data node, which will reset everything and allow it to join the master node.
Note also discovery.zen.minimum_master_nodes is deprecated and ignored in 7.x, and cluster.initial_master_nodes should be removed now that the cluster has formed (and should have been the node.name not the IP address).