1. You can downgrade to Oreo via HiSuite. If the rollback option is not available with your HiSuite ("switch to the older version" or "Earlier version"), you must downgrade to the lower (first) 9.1.0 build version number doing the dload method (flashing Service ROM ) and try again with HiSuite. sometimes better results are with the Chinese version:
2. A) using HCU-client or DC-unlocker (you have downgrade to Oreo build number with google patch april-june 2018 via dload method)
B) for free (that can be done on 9.1.0 ) using potatonv method, Guide
in both cases, you will get an BL unlock code.
How to unlock:
- In Developer options in your phone enable OEM unlock option (with this setting you will get "FRP unlock" status in fastboot mode, it's very important).
- Connect your phone in fastboot mode to the PC and run command
fastboot oem unlock xxxxYour_Codexxx
3) Download TWRP
- in fastboot mode run command
fastboot flash recovery_ramdisk xxTWRP'sNamexx.img
(e.g. fastboot flash recovery_ramdisk TWRP-9.1.img)