I made a tutorial, downloaded the 3 files from XDA forum including the MSM tool to flash
Link:
https://forum.xda-developers.com/oneplus-7-pro/help/how-to-convert-tmobile-op7-to-t3935947
Link:
https://forum.xda-developers.com/oneplus-7-pro/help/how-to-convert-tmobile-op7-to-t3935947