Access oneplus6 stock firmware link inoperable

  • OnePlus 6 Oxygen OS 5.1.5 Android 8.1 Oreo – Link

Trying to un root my One plus 6. Trying to get TWRP from your site (as recommended by T-mobile) I keep getting an error - “Page unavailable”.

What do I need to do to un root my Oneplus6? Thank you