Explanation
Round robin is a load balancing technique that distributes requests among multiple web nodes in a circular order. It ensures that each web node receives an equal amount of requests and improves the availability and performance of the web application. Verified References: [Round robin], [Load balancing]

Port 443 is used for HTTPS, which is a secure version of HTTP that encrypts the data between the web server and the client. Port 3389 is used for RDP, which is a protocol that allows remote desktop connections to a server. These ports should be allowed for a secure web server that can be administered via RDP.
References:
CompTIA Server+ Certification Exam Objectives1, page 15
Common Ports Cheat Sheet: The Ultimate Ports & Protocols List2

Kerberos is a network authentication protocol that uses tickets to allow clients and servers to prove their identity to each other. Kerberos relies on accurate time synchronization between the parties involved, as the tickets have expiration dates and timestamps. If the system clock of a Linux server is off by more than five minutes from the LDAP server or the domain controller, the Kerberos authentication will fail and generate errors. A local firewall is unlikely to block authentication requests if the server can communicate over the network and is configured correctly. The users' passwords are not relevant if they are known-working LDAP credentials. The server has access to the LDAP host if it can communicate over the network and is configured correctly. References:
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/identity_management_gui
https://www.ibm.com/docs/en/aix/7.2?topic=authentication-kerberos-time-synchronization