Before you download anything, you must understand that the Huawei HG520b has several hardware revisions. Installing the wrong firmware will brick your device (turn it into a useless piece of plastic).
Even with a verified download, things can go wrong. Here’s how to recover.
Connect via Ethernet
Access the Web Interface
Navigate to Firmware Upgrade
Start the Upgrade
Post-Upgrade Steps
You will not find official files on Huawei's consumer site. They have been removed from the global support portal. However, five verified, safe sources remain.