MongoDB addShard doesn't add a new shard to the cluster

I have a MongoDB cluster of 2 nodes. The 2 nodes are hosting each:

• one config server
• one shard server
• one mongos instance

When I connect to the machine 1 on a mongos and execute:

> sh.status()

I can see in the sections shards:

shards
[
  {
    _id: 'replicaSetShardServer1',
    host: 'replicaSetShardServer1/machine1:port',
    state: 1,
    topologyTime: Timestamp({ t: 1669040465, i: 2 })
  }
]

But I can't see the machine 2.

So I follow the procedure to add the machine 2 to the replica set.

1. Connect on the mongos instance
2. Execute

> sh.addShard("replicaSetShardServer1/machine2:port

It returns :

{
  shardAdded: 'replicaSetShardServer1',
  ok: 1,
  '$clusterTime': {
    clusterTime: Timestamp({ t: 1675869627, i: 1 }),
    signature: {
      hash: Binary(Buffer.from("323787680faa463455c0aea72d3d9c722575dfda", "hex"), 0),
      keyId: Long("7168472318795055129")
    }
  },
  operationTime: Timestamp({ t: 1675869627, i: 1 })
}

Then I execute the sh.status() again but I still have one shard only (the same as above) indicated in the shard section, therefor I assume the shard isn't taking into consideration.

Am I doing something wrong ?

The result of the command db.adminCommand("getShardMap").map is

{
  replicaSetShardServer1: 'replicaSetShardServer1/machine1:27018',
  config: 'replicaSetConfigServer1/machine1:27019,machine2:27019'
}

I faced 2 issues:

1. I had to use the rs.add("replicationSet/node2:27018") to add the node to the replication set.
2. I had to add a user for the shard server replication set in order to use the rs.add("replicationSet/node2:27018")

For the first point, the missing part of the equation was that I had to firstly add the shard server to the replica set shard server and then call add shard. No error was prompted to the response of the addShard() call.

For the second point, in a mongoDB sharded cluster, there is 2 kind of users.
One that is created through the mongos and used to deal with data and collection operation (query a collection, write to it etc..).

And there is the shard local users, those are created connecting directly to a mongod instance. Those are used for administrative tasks.

https://www.mongodb.com/docs/manual/core/security-users/#sharded-cluster-users