[OnePlus 8 Pro Questions & Answers] Zilch163:
For some weird reason though, the "Telefonica_UK_Commercial" profile via PDC wouldn't give me 5G. Upon further investigation, I found that this profile + the carrier_policy.xml - didn't have the right config for my IN2020 phone.

Subsequently modified the carrier_policy.xml file further and BOOM! 5G is back. Researching the carrier_policy XML further - instead of specific band enabling per the previous config - I edit it to be 'hardware' so that whatever the hardware supports, it will be made available.

Anyhoo, long story short it's all working again thanks to you wonderful people :)

Summary of steps taken:

Prerequisites:
* Need ADB suite available on PC & enabled "ADB debugging" on phone via developer options
* Need QPST suite of apps installed on PC (needed PDC tools + EFS explorer)

Process:
- Open command prompt on PC & navigate to where your ADB suite is located

- Type command without "":
"adb devices" (I like to do this to ensure adb can speak with phone/computer)

- Should display:
List of devices attached
<if working, your device should be listed here (else, it'll be blank)>

- Type command without "":
"adb reboot ftm"
--> This puts phone into field test mode

- Type command without "":
"adb shell"
--> Creates dialogue for shell command on phone

- In the shell (OnePlus8Pro:/ $ ) type command without "":
"setprop sys.usb.config diag,diag_mdm,qdss,qdss_mdm,serial_cdev,dpl,rmnet,adb"
--> This opens up ports that we used to be able to in Android 10

- Phone Screen is still blank - but it's OK.

- Start up PDC Tool in Admin mode on PC

- Click drop down, select phone connection (there's only 1 in the list)

- Do the profile activation stuff for Telefonica_UK_Commercial:
- Look for "Telefonica_UK_Commercial", right click and SetSelectedConfig for both Sub0 and Sub1
- Click on the above profile again and click on Activate
- Do not restart your phone

- Don't close PDC yet (don't know if it make a difference but I didn't close)

- Open EFS Explorer (I opened in Admin mode too)

- Navigate to "\nv\item_files\ims\"
- Replace ims_user_agent file with one supplied
--> The supplied one has Telstra VoLTE Qualcomm as header instead of Telefonica

- Navigate to "\policyman"
- Replace carrier_policy.xml & carrier_policy.xml_Subscription01 with ones supplied.
--> ** Caveat, I further modified the carrier_policy.xml file as it wasn't enabling 5G for me on my IN2020 (Android 11) Oneplus 8 Pro.
--> ** I changed the nr5g_sa_bands base & nr5g_nsa_bands base values to be "hardware" rather than specifically band n78.
--> ** This changes config to allow whatever 5G bands the hardware has available rather than specific bands
--> ** You can't enable bands that the hardware doesn't support anyway so makes sense to enable all
--> ** This gave me the needed 5G access with Telstra

- Type command without "":
"adb reboot"

After phone boots back up, 5G + VoLTE + VoWiFi are all intact and in working order!

Mate, you're a freaking legend. (Magisk Canary root: IN2020 / Android 11 / 11.0.2.2.IN11AA / BOOST network)

I got abit confused with the "carrier_policy.xml_Subscription01" part as it wasn't in any of the files you provided, so just assumed I had to just copy and rename the "carrier_policy.xml".

Also EFS explorer took a little longer than I expected to start up (~1 minute) just incase anyone thinks theirs crashes and close it prematurely.

Worked like a charm.

Cheers