OnePlus is rolling out the Oxygen OS update version 5.1.11 to OnePlus 6 devices. This update brings optimized network connnection, Wi-Fi stability, screen flickering issue in daylight, login to device by just inputing the pin without tapping the tick mark sign and brings in other improvements to the phone. On the imaging front, this update also improves the HDR mode.
OnePlus is rolling out the update in a staged manner and not all devices have received the same. The broader rollout will happen in a few days after ensuring that this update is stable and does not cause any issue. Hence, the VPN method to get the update by changing the country does not work here. All you can do is keep hitting the update button daily and wait for the OTA.