Hmm.
So, you have the bridge device making a TCP connection to Mango, then Mango doing Modbus out the virtual port connected to that server socket?
Are there any options on the GSM-RS485? Specifically, are you using TCP or TCP with keep-alive? I would recommend going with keep-alive, as I would speculate the bridge is closing the connection, perhaps. Is there more to the stack trace in the log?