Are you sure this is tmobile version ? Because all imei numbers for tmo op8 that i have seen starts with 9900. Yours isn't the same. Not sure what is it ...
My phone is IMEI changed because of heavy (Pakistan telecom authority) tax on imported phones.
Aoa, did you get both of your IMEIs changed? By default they patch only 1st IMEI but seems like you got your 2nd one patched as well after converting the phone?
1. First of all unlock bootloader and u also have permanent sim unlock from t-mobile.
Brother after flashing eu10.5.9 just update your OnePlus to latest 184.108.40.206 (because dual sim compatibility is only available in android 11/oxygen 11) and then lock the bootloader after locking your phone will rest and u will see dual sim compatibility after setup.