Hi @Nino_I ,
As mentioned from the documentation, you Need to create user in the primary of shard to perform operation directly on the shard.
These shard local users are completely independent from the users added to the sharded cluster via mongos . Shard local users are local to the shard and are inaccessible by mongos .
Direct connections to a shard should only be for shard-specific maintenance and configuration. In general, clients should connect to the sharded cluster through the mongos
BR