There could be many reasons. Firewall on the server, the name of the server is not recognized in your network (dns). You can try logging in by using server’s IP address.
Domain name is not required for login. You should be able to login using just IP address, but it is better to have a domain name and login with it. If you can’t even login with IP address, then i would check firewall and selinux settings on the server. Try disabling firewall and selinux for a test.
I believe centos 7 uses firewalld instead of iptables, and is enabled by default. Youll need to disable it. or create firewall rules to allows the traffic.