I am using the Ubuntu 18.04 MongoDB one click installation droplet. I have setup three droplets, and want to set up a replica set.
I was trying to follow some tutorials:

here

and here

I am stuck at setting a bindIp, and whenever I add the IP of my other droplets to bindIp, mongod won’t start, failing with this error:

Mar 31 15:52:01 db3.muhive.com systemd[1]: mongod.service: Main process exited, code=exited, status=48/n/a
Mar 31 15:52:01 db3.muhive.com systemd[1]: mongod.service: Failed with result 'exit-code'.

To be clear, this works:

# network interfaces
net:
  port: 27017
  bindIp: 127.0.0.1

but this doesn’t

# network interfaces
net:
  port: 27017
  bindIp: 127.0.0.1,db1.munhive.com,db2.munhive.com,db3.munhive.com

Pinging each of those individually does work though. How do I set this up? Do I need to configure a private network?