I downloaded it on a burner laptop—just in case. 1.8 GB. The archive opened instantly. No prompt. No password wall. Inside were the scatter files: system.img , boot.img , recovery.img , cust.img . The userdata partition was deliberately missing.
Then I found it. A buried thread on a Russian firmware forum from 2019. The post title was simply: "Huawei TRT-L21A Flash File Without Password." Huawei Trt-l21a Flash File Without Password
For the first time in 72 hours, the red LED stayed solid. I downloaded it on a burner laptop—just in case