MongoDB by default take the hostname when creating replica set. In a replica set, the node communicate by that hostname. If the hostname wasn’t resolveable then they will not be able to talk to each other. A way to solve them is using ip address or some public/private DNS as hostname.
Changing hostname is easy, by following these steps.
cfg = rs.conf(); cfg.members.host = "newhostname.tld:27017"; rs.reconfig(cfg);