OPTA ModbusTCP starts and stops after a some seconds

I have the Opta WIFI working with the IoT cloud and exchanging some variables.
I have the RTU working with some slaves working fine.
However when I try to use the Ethernet port to communicate with my PC running the ModbusTCP as slave it starts working just fine but after some seconds the data exchange hangs. I have opened Wireshark and I can see all the breakdown of the ID, function code etc and also the data. I can also see the same using the Modbus slave simulator, but after some seconds the Opta working as a client/Master stops working and I have to click reboot the Opta for it to start working again.
Any one have faced this issue?