Mongo DB 3.2 Authentication - mongodb

Question

I've just followed this guide on setting up Auth with Mongo DB, as well as this guide to get a user set up as an administrator.
Running mongo > use admin > show users prints the following:
{
"_id" : "admin.root",
"user" : "root",
"db" : "admin",
"roles" : [
{
"role" : "root",
"db" : "admin"
}
]
}
After this, I run the server again with --auth and use the following command:
mongo -u "root" -p "xxx" --authenticationDatabase "admin"
This prints the following:
MongoDB shell version: 3.2.19
connecting to: test
2018-03-29T15:52:32.329+0200 E QUERY [thread1] Error: Authentication failed. :
DB.prototype._authOrThrow#src/mongo/shell/db.js:1441:20
#(auth):6:1
#(auth):1:2
exception: login failed
Trying to run this without the --auth parameter lets me log in just fine.
The --auth parameter also gives me the following output in the server console:
I ACCESS [conn1] note: no users configured in admin.system.users, allowing localhost access
But I'm actually unsure about why it isn't picking up any root/admin user I create. When trying to connect with Robo 3T, the terminal prints the following:
I NETWORK [initandlisten] connection accepted from xxx:44924 #2 (2 connections now open)
I ACCESS [conn2] SCRAM-SHA-1 authentication failed for root on admin from client xxx ; UserNotFound: Could not find user root#admin
I NETWORK [conn2] end connection xxx:44924 (1 connection now open

Answer 1

Solution by OP.
Issue fixed by following this article.
It seems that, despite using --auth when connecting to the server, by not commenting out the line bindIp: 127.0.0.1 and adding authorization: 'enabled' to the security section in /etc/mongod.conf, I was only allowing access to the local machine - the server itself. The error messages could have been worded a bit better, but that's security. I guess.
Whilst this was a very silly oversight, no documentation I had previously looked at had covered this issue.