Multiple Customers WANs

I want to be able to monitor multiple customers with different internet addresses from 1 location... but, some of these customers have from 10 - 30 devices each. I The problem is I put in a wan ip address as the device I want to monitor... then it seems to stop right there. I am guessing that OpManager has limitations where it can not go into a NAT`ed network.

example: I am in City A trying to monitor City B so I Add Device 4.18.4.21 | Router WAN IP = 4.18.4.21 Lan IP = 192.168.100.1 | Server A = 192.168.100.10 Server B = 192.168.100.11 Printer A = 192.169.100.20

OpManager seems to only be able to monitor the router and can not discover the other devices within this network

I do NOT want to have to create a VPN to every one of my clients

So I am asking do you guys have a probe or some sort of software I can install onto one of their servers that can go out to each of the workstations and then send the data back to my OpManager server via the internet?

I also do NOT want to install OpManager onto every one of my customers networks LOL.

Thank you in advance!

- Brandon