Plot twist! OnePlus just introduced a global SIM-free data service with the latest Oxygen OS beta. The company says it should work in "most" countries and regions, though we don't have a list yet. The service is called OnePlus Roaming and lives in the Settings, Wi-Fi & Internet. From there you can purchase packages or activate the ones you've bought ahead of time. The list of data packages and their pricing will differ depending on the country. OnePlus Roaming is a SIM-free data service Note that the service doesn't support voice calls, this is data-only (not a huge issue...
Source: https://www.gsmarena.com/oneplus_roaming_announced_a_global_simfree_data_service-news-34301.php
