I had problems which I was not able to add a new node into active replica set. I had followed https://docs.mongodb.com/manual/tutorial/expand-replica-set/ this tutorial on docs. Each was Ok but after I used rs.add(

) , there nothing happen on the member node. I used rs.status() on PRIMARY to check state of replica set but I meet "state" : 6 and  "stateStr" : "(not reachable/healthy)". Please help me, I have not found the solution for 4 days. I repeated the tutorial but having the same result. Thank you so much!