[Sony Xperia 5 II ROMs, Kernels, Recoveries, & Othe] qoro:
The LTE+ icon does usually mean carrier aggregation, but some bands might be disabled for your region, which still sucks.

For example, the stock Sony rom would always stick to LTE 1800Mhz and aggregation would only work on one carrier and even then it would just be 1800+900/1800+ (20+5Mhz) for me. After switching to PixelOS's previous version, my phone started connecting to more bands, it would remain on 2300mhz more frequently too, so I had access to gigabit LTE. Still no Volte support though. It's the same as the stock rom on this version, except carrier aggregation doesn't work AT ALL now... (And still no Volte).

Sony has hidden the enhanced LTE icon and it just shows 4G (gross) all the time. you can see it on the video in the replies here too. You can get pixel ims and Shizuku to fix the network icon issue so your phone shows LTE/LTE+, then you can always know if you're getting enhanced LTE.

After that, you will need a Windows device. It is possible to configure modem files using Qualcomm's tool, usb debugging and EFS file explorer.

You will need to enable enable ADB root using this magisk module (make sure to install it using usb debugging after putting your phone in adb sideload, but have magisk installed beforehand)

GitHub - anasfanani/Adb-Root-Enabler: Magisk Module that allows you to run unauthenticated "adb root".

Magisk Module that allows you to run unauthenticated "adb root". - anasfanani/Adb-Root-Enabler

github.com

Enable all available LTE-features on your Qualcomm device - MT-TECH.FI

This instruction helps you to enable all LTE-features on your Qualcomm devices like 1024QAM DL and 256QAM UL. Some features only supported only with SDX52 and SDX55 modems. Before continuing, I remind you these changes could end your device warranty. Be careful because this can make your device...

mt-tech.fi

Modify Sony Xperia 1 II LTE-A and 5G NR Combinations - MT-TECH.FI

On this instruction i will show to you how you can edit Sony Xperia 1 II 5G NR/LTE-A Bands / Carrier Aggregation Combinations with Qualcomm Software. Sony has limited band and carrier aggregation support per network. You can check in cacombos.com Xperia 1 II supported Carrier Aggregation...

mt-tech.fi

Enabling VoLTE on a Xperia 5 II

Let's save this phone from becoming e-waste.

bleepitybloopity.com

Unlock the band support on X55, X60, X65 equipped Xperia 1 series devices

Enable Qualcomm Diagnostic Mode on various UEs

I used a combination of these tutorials to get it working. Pay extra attention to the one that tells you how to configure the drivers on Windows. When it comes to entering proper diagnostics mode, on the last link, the terminal commands meant for Sony devices only work on stock rooms, since this rom is more similar to that of Google Pixel, you will need the commands meant for Pixel devices (but I tried using Sony ones first and then Google ones, so try them in this combination if directly using Pixel commands does not work).

I can confirm that carrier aggregation started working again, but it was unstable because I did not configure the modem files properly, so I will try doing it again. You can enable VoLTE with this method too, if it is turned off for your region (I still have not done that due to the lack of free time).

Another option is to install PixelOS's previous build, extract the modem files and then upgrade the rom to the newest build and replace the modem files, but the modem crash issue might come back (it does not affect me because I cannot get 5G working at all at the moment, Sony has it disabled for my country of Georgia).

I've only tinkered with the xml files so far, but I will try using the MBN file once I have time... If it can get volte working on my device, I'll be so happy... It might even unlock more bands and 5G for me, I guess we'll have to see.