I have been trying to connect a SCADA system to an Automation Direct PLC through a WR11 cellular modem using Modbus TCP. This same physical system was used previously with an OPC server that used a native protocol to connect to the PLC without issues. I had a port forwarded using the native PLC programming port (28784) and have now added a port forward using the default Modbus TCP port (502). What is interesting is if I initiate a connection from my local subnet to the PLC using the 28784 connection, then the Modbus connection works. It will stay working as long as I am connecting through 28784. If I stop using this connection method then the Modbus connection will work for exactly 40 minutes before dropping out. I suspect that it has nothing to do with the ports, but more the method. I have spent an unbelievable amount of time trying to make this work. Any thoughts would be appreciated.