Just download the full rom from OnePlus web, can boot into recovery mode to flash. Very simple one. That method should work too.
Where to find boot into recovery mode method? And is the full rom the one from the link that I posted? Thanks
Just download & transfer the full ROM to the root of the storage, then click Setting --> System --> System Updates --> Click the cogwheel --> Local Upgrade --> Click the ROM to continue --> Reboot to finish upgrade.
Booting to recovery --> Press & hold Vol down + Power, follow the rest of the instructions --> Clear cache will do.
Upon receiving both my OP6T, I just flashed the full ROM to be sure the phone is clean.