The EMS should work just fine behind a NAT. We tend to run on devices that are within WiFi or 4G networks and so are therefore manditorily behind a NAT.
Do you see any logs from the EMS when you try to connect via RTMP?
Can you try a test to sanity check that the packets are making it to the EMS computer? On that computer, shut down the EMS, and then start a nc listener:
nc -l 1935
If you try to connect again you should see the connection data coming through on the console from this netcat listener.