I presume you are using one of the Moxa MGate series Modbus TCP gateways. We use these devices extensively with Mango and they are fairly easy to setup.
My advice is to ensure you have the correct wiring on the DB9 from your RS232 device to your Mgate. On the Moxa Mgate DB9 pin 2 is RxD, Pin 3 is TxD and pin 5 is ground.
Download and install the Mgate manager software. Configure the Mgate accordingly RTU slave or ASCI Slave.
Ensure you have the correct settings in the Serial section that corrspond to your RS232 device.
Ensure you have the correct IP settings. Save this configuration to the Mgate device. Check your IP settings on Mango as well.
Download a copy of Modscan or similar and test your Modbus connectivity. If you cant see any data then Mango cant either. The default port is 502 (TCP) on both Mango and the Mgate.