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[0].host = "newhostname.tld:27017";
rs.reconfig(cfg);