Hi,
@raxsix you might want to have a look at this thread: MQTT Client Support
There are other people having the same issues on there. I am using Legato 17.09 and it is working fine. Are you using Developer studio?
@lyao If I understand it correctly, clientID takes a string of your choosing as input. In the example code is is using the device’s IMEI number plus “-pub” at the end. So for example 123451612-pub