Honor 7x BND-L21 downgrade issue

[AlexLebul]

Hello, more than a month ago, I updated my phone on emui 8.0 using the official beta program. I would like to return to emui 5.1, but when when I try to flash the FULLOTA file (UPDATE.app) with the official recovery, it fails and gives me an error. So I thought to install the rom with twrp, except that I do not find how to unlock my bootloader. On the site of emui, there is no more the page to unlock it by receiving a key. I would like to know if there is a solution to return to emui 5.1 without unlocking the bootloader, or if there is another way to unlock it without using the official site.