I had the same issue but I tried to downgrade it
I bought used h811, but it has issue of restarting every time I tired to uninstall any app or tried to go to security setting. So I decided to downgrade it (I have no idea about ARB at that time), the device show h811-21y, So I download h811-10n firmware & use lgup to downgrade after it complete 100% it get bricked, and show qualcomm 9008 on the PC. I then open the device and found out it is h810 written on the motherboard.
I tried almost every qfil mbn files present on the internet but failed.
Then I tired this firmware
LGH810AT-01-V21y-310-410-JUN-20-2016-ARB03+0.rar
and use this youtube method to create raw programme & patch
watch?v=RhG02kmSelk
Then I use sd card method as this youtube method suggest
watch?v=kX2Fm1zEx5E
my device show sign of life but stuck in "secure booting error 1009"
At that point I am totally stuck since I does not get any help with this error on the internet