1. First of all unlock bootloader and u also have permanent sim unlock from t-mobile.
2.After unlocking your phone will restart, setup your oneplus 8 and enable developer mode and in developer mode enable "usb debugging" .
Note: You must have working adb/fastboot drivers.
3. now restart your phone to bootloader.
4. Download any android 10 fastboot Rom from xda developer.
"I have tested this : https://sourceforge.net/projects/fastbootroms/files/OnePlus 8/10.5.9-EUROPE-OnePlus8Oxygen_15.E.18_OTA_018_all_2007160043_913306-FASTBOOT.zip/download
4.1. For flashing this rom your phone has to be in fastboot mode.
5. After downloading extract the rom and open extracted folder and double click "flah_all.bat
5.1.connect your phone to computer in fastboot mode and enter "Y"
wait for 5 to 7 minutes, in this process your oneplus will restart to recovery mode but never unplug the cable or use your phone during installation.
Note: i have tested dual sim with dual sim tray.