I did it and it worked successfully... But do not understand why I do not have mobile data after the update, for calls and messages is ok.Yeah this was my first OnePlus and it may well be my last because of the throttling. It really irks me. Like they go so far as to make it impossible to actually disable in OOS12.
Open Magisk, hit uninstall, hit restore images. Install the OTA and before rebooting, install to inactive slot.
Op9 T-Mobile unlocked and with the global rom.