[4qx]

Thank you @MaxPayne. I really appreciate your taking the time to help, no one else I tried asking on different sites helped (and in some cases they were just deleting the post). The setprop commands you provided enabled diag mode, though the phone kept reconnecting after each executed command for some reason. I also tried installing Magisk module twice but I'm not sure if it worked because it doesn't show in Modules tab. Anyway, QFIL, QPST and EFS' NV Tools can all connect to my phone through its COM port now and I attempted IMEI restorations, but none of my attempts worked. Here's what I tried:

Backing up the .qcn in different ways via QFIL, QPST and NV Tools; editing the IMEI using NV Tools and Qualcomm IMEI Rebuilder, then checking that the file actually got edited and contained proper IMEI; restoring the qcn with QPST and QFIL (backup and restore with and without Muti-Sim option). In spite of attempting all these variations, the phone keeps persistently showing the broken IMEI via Settings and *#06# dial code after rebooting. Curiously enough, in case of trying to write the correct IMEI with NV Tools, there's an error reading "NV_ERR_READ_ONLY_NV_ITEM", as opposed to QFIL and QPST which simply show success messages.

Once again... any advice?

[maxpayne]

Thank you @MaxPayne. I really appreciate your taking the time to help, no one else I tried asking on different sites helped (and in some cases they were just deleting the post). The setprop commands you provided enabled diag mode, though the phone kept reconnecting after each executed command for some reason. I also tried installing Magisk module twice but I'm not sure if it worked because it doesn't show in Modules tab. Anyway, QFIL, QPST and EFS' NV Tools can all connect to my phone through its COM port now and I attempted IMEI restorations, but none of my attempts worked. Here's what I tried:

Backing up the .qcn in different ways via QFIL, QPST and NV Tools; editing the IMEI using NV Tools and Qualcomm IMEI Rebuilder, then checking that the file actually got edited and contained proper IMEI; restoring the qcn with QPST and QFIL (backup and restore with and without Muti-Sim option). In spite of attempting all these variations, the phone keeps persistently showing the broken IMEI via Settings and *#06# dial code after rebooting. Curiously enough, in case of trying to write the correct IMEI with NV Tools, there's an error reading "NV_ERR_READ_ONLY_NV_ITEM", as opposed to QFIL and QPST which simply show success messages.

Once again... any advice?

Can you somehow get a QCN backup from a working phone? How did your phone lose IMEI? Have you flashed the latest firmware for your region / build / variant?

[4qx]

Can you somehow get a QCN backup from a working phone?

Sadly, no.

How did your phone lose IMEI?

Wiped something I shouldn't have by accident.

Have you flashed the latest firmware for your region / build / variant?

Yes, 47.2.B.5.38 (Android 9) for SO-02K from ftf.andro.plus

I've been testing a bunch of ROMs: 47.1.F.1.105 and 47.1.B.0.542 (Android 8), HavocOS v4.15 (Android 11) and Lineage 19.1 v1.3 (Android 12), but on those ROMs I wasn't even able to activate Diag mode and open the phone's COM port. Pretty lucky that I just happened to have been on 47.2.B.5.38, otherwise I would have thought that even enabling Diag mode is impossible. Ultimately I flashed back to 47.2.B.5.38 and since several guides online suggested that in order to remove the IMEI write protection some partitions must first be removed, after backing up here's what I did:

Entered twrp to erase modemst1, modemst2, fsg partitions via these commands dd if=/dev/zero of=/dev/block/bootdevice/by-name/modemst1[b] and delete persist and modem, powered off, entered flash mode and flashed the stock ROM's persist and modem via Newflasher v20 (adb's fastboot refused to flash them). As a result, NV Tools responds to every IMEI write command with success, but without any actual result, and QFIL and QPST refuse to backup or restore anything.

I also discovered the following:
- To enable or disable Diag mode, Xperia has to be put in [b]No data transfer USB mode and these commands are needed:
adb shell
su
setprop persist.sys.usb.config diag,serial_cdev,rmnet,adb
- Qualcomm Smartphone Write IMEI Tool and WriteIMEI are equally unable to write in the IMEI

Any suggestions are... welcomed.

[maxpayne]

Any suggestions are... welcomed.

It appears finding an NV backup is the way

[4qx]

Hmm. Is it possible to edit modemst1 and 2 .img files?

[maxpayne]

Hmm. Is it possible to edit modemst1 and 2 .img files?

I don't think this will do much good as we have nothing to compare with. I might be wrong though. Under your Settings screen, what do you have under Software version and Custom version? I think you need to flash the right cust.
SE Tool appears to support this model

[4qx]

Under your  Settings screen, what do you have under Software version and Custom version? I think you need to flash the right cust.

There's nothing labeled Software version or Custom version unfortunately. App info on Settings says version 9, don't think that's what you're after though

I don't think this will do much good as we have nothing to compare with.

I'm open to trying though

SE Tool appears to support this model

Great. How could I use this SE Tool?

[maxpayne]

Great. How could I use this SE Tool?

Its a paid tool . You can look it up

[4qx]

It won't open past the splash, "SETOOL2 SMARTCARD NOT FOUND"