hey everyone wanted to update that i've got 5g+volte+vowifi working on my OnePlus 8 now!! (with Optus). thanks to all the contributors on this thread, it's been a massive help
sharing some of the steps i took/things i learnt along the way:
1. reinstalled the qualcomm drivers here (
https://gsmusbdriver.com/install-qualcomm-usb-driver) as suggested by mhall156 (didn't have to uninstall anything)
2. reinstalled the oneplus drivers provided in the youtube video:
(this seemed to fix the "**** connect to device" error in PDC i was facing previously. Again, didn't have to uninstall anything)
3. this one might be obvious to most but to replace the ims and policy xml files is just a matter of deleting the existing files and uploading the files provided. i kept trying to overwrite the current files somehow - silly me. (also didn't have to change anything in the xml files even on optus network).
4. also if you got an error while your phone was in the "adb reboot ftm" mode, just do a fast reboot by holding the power button and volume up button simultaneously.
5. remember to contact your carrier to enable 5g/volte. after i completed all the steps, it was still not working for me. turns out the 5g/volte services aren't activated by default and i had to contact optus chat to request these services to be enabled
hope this helps someone and i'm off to enjoy what feels like a brand new device now
