As of May 2019 the Hologram Dash has been depreciated and Hologram is no longer supporting it.
Make sure you have activated your SIM on the Hologram Dashboard and that its corresponding device has a LIVE status.
You may need to reload the Logs page in the Dashboard in order to see new incoming messages.
Confirm Serial.begin() is called in your setup() function. The serial port is not available while the Dash is in program mode.