You can find the download link of ZTE Nubia V18 NX612J Stock Firmware ROM (flash file) on this page. The Stock Firmware comes in a zip package that contains the original Firmware, Flash Tool, Driver, and How-to Flash manual.
You can use the ZTE firmware to Upgrade or Downgrade the Operating System (OS) of the device, Unlock or remove the device’s protection, Fix the IMEI-related issue, or Fix the bootloop problem.
File Name: ZTE_Nubia_V18_NX612J_OTA.zip
File Size: 2 GB
How-to Flash (install): Copy update.zip file to the phone SD memory and flash it from the recovery mode.
[*] Backup: We recommend taking a backup of your data before flashing or installing the Stock Firmware (ROM), as flashing the Firmware will wipe the data from the device.
[*] Original firmware: We only Serve the original firmware, which the device manufacturer releases, and we scan each of the firmware using MalwareBytes Premium before sharing it online.