Sorry for my English. It has been machine translated.
On my OnePlus 7 (not pro) Android 11 rooted, it was enough to follow simple steps.
When you have selected options in the sim card menu, connect the phone with a USB cable:
In CMD / command prompt / open as administrator and enter in turn:
(you need to grant root permission to the shell on the phone during this)
adb shell
enter
su
enter
setprop sys.usb.config diag, serial_cdev, rmnet, adb
enter
This opens the full port on the phone for PDC (in QPST)
Now you can open PDC / as administrator /, the phone will be properly detected and you can subdue the region.
Good luck