Unable to flash MR1100-2A1NAS with Generic firmware

I bought an unlocked MR1100 (AT&T-branded). I'm trying to use it with T-Mobile (via TravelData unlimited LTE reseller). I'm using b2b.static as the APN (as instructed). I'm unable to get a data connection.

The actual model number is MR1100-2A1NAS (which I believe is the 2nd gen with Band 14). TravelData tells me that these B14 models do not work with T-Mobile. That seems bonkers because the router still supports all the other T-Mobile LTE bands… 

So, before returning the router, I'd like to try flashing it to the latest generic North American firmware.

I've got FDT 4.6.2.0. I've got the latest firmware as of today (MR1100-100NAS_23113828_NTG9x50C_12.06.11.00_00_GenericNA_05.03.secc.spk). I've got the latest AC785 drivers. 

Running cmd.exe as Administrator, I ran fdt with the spk. It fails after 420 seconds:

C:\Users\IEUser\Downloads>fdt MR1100-100NAS_23113828_NTG9x50C_12.06.11.00_00_GenericNA_05.03.secc.spk
FDT version: 4.6.2.0
Checking modem mode ...
Disabling selective suspend ...
Switching device to boot & hold mode ...
Firmware download failed.
Primary error code: 67 - Firmware update process timeout.
Secondary error code: 0 - Not applicable.
Total time elapsed: 422000 ms.
Press Enter to continue ...

JohnPeng (and others): Do you know / could you share the process for flashing a 2A1NAS to the generic firmware?

Thanks!