I just finished arguing with metro employee about porting a google voice number, to get this thing, and come to a grayed out bootloader option. Please put the info in your computer, yes, it will work, assuming your still compliant with fcc regulations since the last time. Thank you. Rant rant, anyway....
The fix is mentioned here https://forums.oneplus.com/threads/metropcs-nord-n10-5g-bootloader-greyed-out.1403286/
Involves general "debloating" method explained here on xda. Disabling specific carrier apps enables the option in development settings. I'm about to do it now, I'm confident it will work I've fixed other restrictions in the same way with Samsung phones and will post specifics soon including adb commands.
He even puts the commands in a script for us, download the zip from here https://forum.xda-developers.com/t/oneplus-pure-debloat.4157691/
I use Linux, windows should be similar, nothing will break(your not rooted yet) and these will come back anyway since unlocking the bootloader formats data.
extract zip, run commands in the file
7z x Nord_Debloat_Full.zip
bash Nord\ Debloat\ Full.cmd
Find your enabled bootloader option in developer settings, fix it till breaks. HAPPY FLASHING!