I've been using both the Redmi Note 5 and Redmi Note 5 Pro for a couple of weeks now and have found that both the devices shows an error while Adding an account in Developers Settings (Mi Unlock Status).
When you'll try to Add an account in the Developer Setting in order to Unlock your Mi/Redmi Smartphone you'll get a message "10008： 参数错误、 请参考API文档". Well, there's an fix to this issue.
All you need to do is download the Chinese VPN application named "快帆 海外华人听国内音乐看视频玩游戏的中国VPN工具" from the end of this guide. I've already tested tons of other VPN apps, and only the one i mentioned works.
Just Install the above Chinese VPN, you'll see two options below the round power button, just select the former and turn on the first option (as shown in the below image) to make the whole traffic of your applications go through VPN.
Finally, you're good to Add your account to the Mi Unlock Status Developer Settings. Enjoy Unlocking Bootloader.
Download: Chinese VPN