I am installing nagiosxi "xi-5.5.8.tar.gz" on Red Hat Enterprise Linux Server release 7.5 (Maipo).
I followed document for offline installation, however still it is trying to connect to 2600:f03c:91ff:fedf:b821: and throwing message "Network is unreachable"
I have attached the install.log for your reference.
The package you want to install would be found under https://repo.nagios.com/?repo=offline. I would suggest 5.5.11 since it fixes some pretty big issues found in 5.5.8.