Does G900a or G870a work in USB Diag Mode for QPST?

Does the G900a and/or G870a work in diagnostic mode for QPST, or does the locked bootloader prevent this?

I'm trying to test turning on the Tmobile LTE and the extra GSM channels on a G870a but device is not recognized by the computer as soon as I switch to rndis + dm + modem + adb.

Official Samsung USB drivers, debugging enabled, standard reinstallation tricks and such already tried. What am I overlooking? Used the right cable too...

Or is this just a non-starter from the bootloader?

Thanks!