I have aRUT956 and have connected a simple modbusRTU device to it, I have enabled ModbusTCP Gateway on the router, and have configured the speed (9600) and the data format (8,N,1). The unit does work retrieving Modbus data correctly. The issue is that Polling 16 Float registers at 1000ms using a wifi connection (Wifi is client 1 metre from the RUT956, which has factory antennas connected and full wifi strength). In this configuration, the modbus tcp will start to time out, then start timing out with high latency on any packets that do arrive at the destination.
Using the same system with the same data rates, cabling etc. on another brand of gateway, the connection is reliable.