This is the trickiest part. Huawei does not distribute ONT firmware to end-users publicly. You have three legal sources:
Huawei has released a new firmware update for the GPON/FTTH terminal, addressing security vulnerabilities, improving Wi-Fi stability, and enhancing VoIP compatibility. The latest version, V500R021C00SPC180 (or later, depending on your ISP), began rolling out in late 2024.
Because of this ISP customization, the firmware on these devices is rarely "stock" Huawei software. It is modified to include specific ISP settings, branding, and features like remote management. This distinction is the single most important factor to consider before attempting a firmware update.