MongoDB replica set configuration error
The problem is clearly stated: the first replica set member is bound to localhost.

Since with this name the other replica set members could not connect to this instance, you can only have other replica set members on the same host (but a different port for obvious reasons).

Chances are that the MongoDB instances at DO are bound to localhost by default. You need to bind them to a different IP address using the net.bindIp configuration option. Please keep in mind that this makes you MongoDB instance accessible to everybody being able to connect to the chosen port. You should enable authentication if a MongoDB instance is bound to a different IP than localhost.