Fixed it:I have tried the procedure several times, and I am getting the same 2 errors every time:
fastboot flash reserve reserve.img
Sending 'reserve' (259328 KB) OKAY [ 5.521s]
Writing 'reserve' FAILED (remote: '(reserve_a) No such partition')
fastboot: error: Command failed
fastboot reboot fastboot
results in the phone rebooting into the T-Mobile branded OS, with broken modem.
If anybody seen this kind of errors, please help!
First error can safely be ignored;
For the second one: Allow the device to fully boot. Enable USB debugging. Make sure it can be seen by adb (adb devices should list it). Reboot via adb (adb reboot fastboot). Run the second part of the flash-all script.
Thank you to all the contributors to this topic.