Blinking white is not an indication of a working connection and in AUTOMATIC mode this means no application code execution till you see breathing cyan.
Blinking blue would suggest a problem with the SIM (e.g. not inserted, bad connection, ...)
I'd try leaving your AT commands away for the test to see if they trip up the connection with treading enabled.
Are you using the Particle SIM or 3rd party?
There is a "double post" or spin-off of this thread at
Problem with using SYSTEM_THREAD on Electron?