Passive Monitoring with Nagios / Icinga - How to monitor external hosts correctly?

Good morning,

Our
goal is to monitor many external clients with a open source monitoring solution like
Icinga or Nagios..

Because most of our
customers have multiple devices but only one dynamic public IP, we are still looking for
one solution which works for all of our clients. The amount of clients and networks is
too high that we can use VPN to connect them with Icinga. It would fit if all clients
send their check reports to the icinga host. Many other monitoring solutions, such as
GFI MAX simply let the clients report through TCP 443 or TCP 80 (failover). If an
interval has been interrupted, the monitoring server will mark the client with a
critical failure.

Here some more
information about our internal infrastructure:
- Icinga Core 1.11.1 /
Nagios
- static IP for monitoring server
- Endian
Firewall

At the client side:
- Windows
devices with NSClient++
- no static IP
- NAT and firewall are
configurable

Based on my information
can you suggest a solution to let the clients send their information to the Icinga
server, please? Besides is it even possible to realize this scenario without VPN, static
IP or dynamic dns?

THank you for helping me
out!