Download the rollback firmware, push it to the phone with:
adb push 'fulldowngrade_wipe_MSM_17819_181025_2315_user_MP1_release.zip' /sdcard
Go to Settings > System > System Updates
Push the Gear icon followed by 'Local upgrade'
Select the fulldowngrade package
Confirm
Wait until finished
Reboot (takes a while with spinning circle logo)
Enable developer mode
Enable USB debugging
Use UBPorts installer to flash...
Edit2:
To recover back to Android/or install Android9 via terminal:
Download TWRP3.3.1
newer versions of TWRP did not work for me, but you might test and give feedback...
flash it with:
fastboot boot 'twrp-3.3.1-2-enchilada.img'
Thanks for this guide!
The links to the ROMs in README are not working anymore, a search in the Internet gave me following links:
oneplus 6 official build 10.3.x
oneplus 6 Android 9 ROMs found here: OnePlus 6 OxygenOS 9.0.9 | Download OnePlus 6T OxygenOS 9.0.17 | Download
and the Android 9 rollback firmware is found here
Edit:
Download the rollback firmware, push it to the phone with:
adb push 'fulldowngrade_wipe_MSM_17819_181025_2315_user_MP1_release.zip' /sdcard
Go to Settings > System > System Updates Push the Gear icon followed by 'Local upgrade' Select the fulldowngrade package Confirm Wait until finished Reboot (takes a while with spinning circle logo) Enable developer mode Enable USB debugging Use UBPorts installer to flash...
Edit2:
To recover back to Android/or install Android9 via terminal:
Download TWRP3.3.1 newer versions of TWRP did not work for me, but you might test and give feedback...
flash it with:
fastboot boot 'twrp-3.3.1-2-enchilada.img'
Download original Oxygen Android 9 ROM
push this image to the device with:
adb push OnePlus6Oxygen_22_OTA_025_all_1809150101_35981ba3cecc19e.zip /sdcard
in TWRP on the phone select: INSTALL and there select the pushed .zip file this might take quite a while...
Now you are ready to use Android (first boot takes ages...) or to start installing UBports again.
BTW: pw is: phablet