Trying to Monitor a Windows Server using Nagios over WAN

I am running Nagios from my LAN on a VM running Ubuntu Server. I am trying to monitor a Windows server located and via the Internet. I set up a public IP in my NAT which forwards to my Nagios server as needed.

I cannot seem to be able to ping the Nagios server from the Windows server, nor can I reach the Windows server from the Nagios server (can only ping the Windows server if I disable the public profile in the Windows Firewall).

I'm not sure where/how to proceed. I read that ports 5666-5667, 1248-1249 and 12489 need to be forwarded, but I do not know if that would be on the Nagios server, Windows Server or all of the above.