I have recently also run into a similar issue with my Particle devices.
Have you tried sending one character to the device?
For me sending one character was enough to get the output of the device show up in Serial Monitor.
Some further investigation revealed that the trouble came from the way how the virtual DTR/RTS lines are treated by Serial Monitor (depending on selected target board).
If sending one character starts the output, you could try selecting an Arduino board as build target - not for actually building but only to open Serial Monitor with the Arduino settings for DTR/RTS.