According to https://bugzilla.redhat.com/show_bug.cgi?id=1466709 Nagstamon for Fedora is still stuck on version 1.0. To allow Fedora users to get updates automatically there is now a small repository available via /files/repo/fedora/nagstamon-testing.repo.
Get it like:
wget /repo/fedora/nagstamon-testing.repo --output-document=/etc/yum.repos.d/nagstamon-testing.repo
dnf install nagstamon
This repository currently is not GPG signed – just before someone is asking.