Ok, this is kind of driving me nuts. I have two droplets, one running Elasticsearch (E1) and one running Mongdb (M1) with authentication enabled. I can log into the M1 droplet from E1 running a mongo shell at the command line and then db.auth into my collection. What I can't do is connect using a command such as:
mongo -u <USER> -p <PASSWORD> <HOST>:<PORT> --authenticationDatabase <AUTH_DB>

I know the user, password, host, port, and auth db all work and can be reached. What am I missing?