TRB140 does not create a virtual COM port when connected via USB. Rather, it creates a virtual network adapter which is usually utilized by Windows/Linux/MacOS.
The device you're looking for is most likely TRB240 or TRM250, as they act as modems, and create an AT command COM port, which can be used to directly manipulate the modem and send the SMS messages.
Other devices that could be used for this as well are TRB142, TRB145, RUT956, and other devices with serial or USB host ports. This way, the serial cable could be plugged into the PC directly, and the router/gateway could be set to partial or full control. In partial control, the modem will be controlled by our device, while in full control, the serial connection host will be in full control. More information can be found here.
Let me know if any more information is needed!