Hi @lschmid. First, apologies for the delay in responding.
When you say that you have verified that data is being sent and is stored at ".monitoring-es6-YYYY-mm-dd" indices on the monitoring cluster, do you mean that you have just verified that the indices exist or have you verified that the documents stored inside do originate from the production cluster?
Typically, the configuration is pretty straightforward but here's an overview just in case you missed anything: https://www.elastic.co/guide/en/kibana/current/monitoring-data.html