Well I manage to fix my redmi 4x Volte on my Yes provide in malaysia by. This method though hope can helps you guys out. I did manage to unlock the bootloader first, then flash in custom twrp recovery via fast boot, erase all in advance under twrp before you flash any custom ROM, then continue flash custom ROM, after when it boot up first time, reboot back to twrp recovery, flash the modem .Bin from miui 8.2.9. your Volte will works after when your phone boot up. Just remember to make sure the modem u use is from miui 8.2.9/8.2.10.