With fastboot program you can upload/copy a SD card image to the phone/tablet. This upload/copy can be risky because there is no verification it you actually upload the correct image to the correct phone US/EU model. If you use the alternative update via recovery method there is a verrification if the officially ZTE is updating the correct compatible phone. So be warn that after fastboot method your phone may not be correctly working again.

Setup the needed software. ZTE open use the same android tools 'adb and fastboot'

The normal procedure is to restart the phone to get into the fastboot.
Ignore 'sudo' command in this sample, if you are windows user.

Check if device are connected: see also USB connection tool adb  You may need to use 'sudo adb kill-server' and ' sudo adb start-server' to get the connection going.
sudo adb devices
List of devices attached
roamer2    device
Will response with 2 lines. It have found the device. Which is a good sign. That the 'Remote debugging' setting is enabled.

Check if fastboot is working
sudo fastboot devices
No response is coming back from fastboot. What is wrong?

Try to restart
sudo fastboot restart
< waiting for device >
Keep waiting forever. What is wrong?

The problem is that fastboot can only communicate with the phone at booting/startup of the phone.

To start up the phone in fastboot mode use:
sudo adb reboot bootloader
The ZTE Open phone will restart. And depend on the firmware it will stays in the bootloader mode. Or start up again till the login menu.

Fastboot can now see the device it is connected to.
sudo fastboot devices
ROAMER2 fastboot
When the phone is starting up. You should keep sending this 'sudo fastboot devices' every second till a response is available.

Upload all the images one by one
sudo fastboot flash boot boot.img
sudo fastboot flash userdata userdata.img

sudo fastboot flash system system.img
sudo fastboot flash recovery recovery.img
sudo fastboot erase cache
sudo fastboot reboot

Example firmware with fastboot disabled

sudo fastboot erase cache
erasing 'cache'...
OKAY [  0.532s]
finished. total time: 0.532s
sudo fastboot flash userdata userdata.img
sending 'userdata' (57362 KB)...
OKAY [  4.789s]
writing 'userdata'...
FAILED (status read failed (No such device))
finished. total time: 10.117s

Everything with fastboot works but the writing will fail if the fastboot is disabled. If this happens then update first your firmware with the latest official ZTE firmware. ZTE Open firmware

Example firmware with fastboot enabled

sudo fastboot erase cache
erasing 'cache'...
OKAY [  0.531s]
finished. total time: 0.531s
sudo fastboot flash userdata userdata.img
sending 'userdata' (57362 KB)...
OKAY [  4.782s]
writing 'userdata'...
OKAY [ 10.390s]
finished. total time: 15.173s
The writing of the img file will result with OKAY

Use a batch file to automate the update procedure.

adb reboot bootloader
fastboot flash boot boot.img
fastboot flash userdata userdata.img
fastboot flash system system.img
fastboot flash recovery recovery.img
fastboot erase cache
fastboot reboot

Just put all the command in a text file and execute it.

Windows user:
Delete the first line #!/bin/bash

Linux user:
start the batch file with 'sudo'

 sudo ./
< waiting for device >
sending 'boot' (4600 KB)...
OKAY [  0.385s]
writing 'boot'...
OKAY [  0.822s]
finished. total time: 1.207s
sending 'userdata' (57362 KB)...
OKAY [  4.787s]
writing 'userdata'...
OKAY [ 10.394s]
finished. total time: 15.181s
sending 'system' (124183 KB)...
OKAY [ 10.350s]
writing 'system'...
OKAY [ 21.613s]
finished. total time: 31.964s
erasing 'cache'...
OKAY [  0.531s]
finished. total time: 0.531s

finished. total time: 0.001s

Notice there is < waiting for device > line. This is from the first fastboot command. The phone is restarting.
< waiting for device > is less than 5 second on screen before continue with rest of the flash procedure.