I've been on this same issue (trying to unlock my T-mobile variant of same phone, Oneplus Nord N10 5G) for quite some time now.
What I have successfully done, as prerequisites, are:
1. Unlocked Bootloader
2. Rooted Phone with Magisk
3. Flashed stock rom: Global version
4. Backed up some necessary 'critical partition' files such as boot.img, recovery.img, modem.st1, modem.st1, modem_a and modem_b.
Many people suggested deleting the modem_st1 and modem_st2 files would unlock the sim network.
Well, after the backup, I was able to issue an erase command (which only works in Fastbootd) to erase/delete the 2 files, modemst1 and modemst2.
Unfortunately, that hasn't unlocked the sim network yet. The only thing is has done is to stop the phone from prompting that the sim network is locked and .......
So, that's where I am at with same issue y'all have.
Just thought I'd share to help someone