Ok so Mobile should recognize that comport. From the Mobile GPS settings, have you tried every Baud rate for com 15? Start there first.
If so, then I would start by going into the Device Manager from the Control panel. Open the Ports (Com and LPT) Do you see listed the comport for this USB to serial converter. Verify here what comport is shows. We can try and open that drive and see under the Port Tab if changing to a comport lower then 10 (if available) may help. Remember you need to change the GPS settings in Mobile to this matching comport selection then.
I still suspect that the receiver may not be outputting NMEA causing the issue. Thus why the main computer connects to the receiver to run this Ag Remote program to reconfigure the receiver to output NMEA data strings. This is where your Trimble Hardware Dealer comes to play.