The maps use an Elastic hosted WMS server, so if you don't have a connection to the internet you won't be able to see the maps. You could potentially run your own WMS server and use this with the Regionmap, or create a firewall exception for the Elastic hosted WMS server.Also check this out: Using an offline map in kibana
There recommend way is to setup your own server, see https://www.elastic.co/blog/custom-region-maps-in-kibana-6-0 5 for a tutorial.