The embedded router mode is enabled, the shards must be added to the cluster via addShard command from a router port that already knows the config connection string, that is, the router port of a config server node.

If ShardingTest deploys a sharded cluster with the embedded router mode and config shard, this is not respected and the testing infrastructure will try to add shards from a router port not yet initialized.

This is respected when config shard mode is disabled.

Reference: here