https://www.theverge.com/2017/12/10/16758728/oneplus-5t-netflix-amazon-prime-video-hd-streaming

The OnePlus 5T is unable to stream content from Netflix, Amazon Prime, Google Play Movies, and other video services in high-definition, despite its HD-capable display and powerful hardware. The deficiency was noted in a thread on the OnePlus forums and points to lack of support for Widevine Level 1 DRM, which many services require for HD streaming. The 5T currently supports Widevine Level 3, which only covers standard definition streams. The Verge has confirmed that both the OnePlus 5T and its predecessor, the OnePlus 5, are currently unable to stream Netflix or Amazon Prime Video higher than SD resolution.

A company spokesperson says that a future update for the 5T will enable Widevine Level 1, but did not elaborate on why it was omitted from the start. The spokesperson was unable to provide a time frame for when the update will be made available at this time.