You have another thread for the same problem, where you seem to have got it working:
What is not clear to me is what you are trying to achieve.
Should the RHEL7 machine be a master or should it be a satellite/agent connected to the Ubuntu master?
Please be more specific on that, because the two threads contradict themselves.
Another thing the comes to my mind when seeing RHEL: Is SELinux enabled? If yes, is the icinga2-selinux package installed?