It appears Nagios XI is a commercial extension of Nagios. Fedora is a non-commercial bleeding edge distribution of Linux. Good for seeing what the latest/greatest is but not a good choice for running Production services.
Why do you want to use this commercial product on a non-commercial Linux that you'll have to upgrade within 6 months? That is to say why do you want Nagios XI (as opposed to Nagios) in the first place? If you need Nagios XI why do you not intend to run it on a supported distribution like RHEL?
You should either use Nagios (not Nagios XI) on Fedora or think about using RHEL (RedHat Enterprise Linux) for your Nagios XI install.
Fedora as a project is supported by RedHat but has no direct end user support from RedHat. RHEL on the other hand is a subscription based distribution that does have end user support. RHN = Red Hat Network and is where the support for RHEL is done. It has no meaning for Fedora. Note that RHEL subscription costs money.
Both use yum, however they use different repositories.
You MIGHT be able to run Nagios XI on Fedora but my guess is that it isn't supported by its makers to do it. You might want to give the folks at Nagios XI a call to see what they have to say. They have a number listed on their site: