Modbus TCP over cellular modem

I have been battling for some time an attempt to connect to an Automation Direct PLC’s using Modbus TCP. These items are connected over a cellular network with the Digi WR11 at the PLC. I can connect through native Automation Direct IP methods no problem (for example: PLC programming software, Kepware OPC server that uses native AD), but Modbus TCP seems unable to establish a connection. What’s interesting is that if I use some other connection method (IP) to establish a connection from my local subnet, I can connect to my device over Modbus TCP. As long as I keep my IP connection going, the Modbus TCP connection will also maintain. If I end my IP connection, the Modbus TCP connection will end exactly 40 minutes later. I have played with timeouts and used several different Modbus TCP drivers (Modbus Poll, Wonderware) and they all act the same. Any thoughts?

Ive dealt with thousands of cellular modems and have never seen that behavior. Really all that should happen is you forward tcp traffic from the wan on port 502 to tcp port 502 on the lan side of the modem which would be your AD devices ip address… The protocol shouldnt come into play at all. Can you post some screenshots of the config in your modem?

The setup is pretty simple and it does work as long as it’s initiated with the native AD protocol!

The master SCADA is going directly out over the internet through a local router. I am interested in the fact that this only works if I initiate with another device within my local subnet. That would suggest to me that maybe there’s a WAN IP recognition that needs to take place at the modem. Maybe not just port forwarding, but perhaps an IP forward as well. I’m not an expert on such things, so just throwing out ideas that may be meaningless.

Most cellular modem have a “Keep Alive” setting that should be turn on if the modem is to NEVER disconnect from the ISP.