[Xiaomi Mi A1 Guides, News, & Discussion] Jemec26:
Hello guys,

As I have stated on my Q&A thread I was going to be posting a tutorial in order to bring LTE back for those who had lost it. I will give you some background in order for you to know how I lost it and then I will explain how did I get it back:

How did I lost it:

I had bought a Xiaomi Mi A1, and had given my previous Redmi Note 5 SD636 to my wife, for reasons that don't care at this time haha. So, I get it and I am on carrier Tigo Colombia, which uses band 4 AWS-1 . Everything was working as expected, I had my 4G LTE as a regular user. I had installed the gcam, unlocking bootloader and then relocking it, flawless. Everything was working perfect.

The issue came, when I decided that was time to try one of the AOSP Pie roms. I decided to try Simplix Blue, which called my attention due to not being available on RN5 the device that I used to own. I am pretty sure, that has nothing to do with the rom, for reasons that I will explain later on, but more fault of the SD625, boot.img and modem of the device. So I flashed the rom, as I usually do, following instructions, installing TWRP, changing slot, bla bla bla.. you already know this. So I booted up the rom, and it was on 3g, seemed strange, but on the initial set up I thought myself it was more of configuration on the network side. So, as soon as I finished the set up, I went into settings and selected LTE/WCDMA, did not work. Selected only LTE, did not work. Dialed #*#*4636*#*#, choose LTE network, did not work. So I said, this is more of a ROM problem then. Installed Arrow OS, same issue. Started getting strange, so I said, maybe it is a stock rom problem on my device, so I downloaded Stock ROM and flashed. Surprisingly, did not work either.

So I started to look around for solutions, wrote on telegram group and got ignored, as usually happens on that group, and I posted a Q&A thread here, which you can find here:

https://xdaforums.com/mi-a1/help/lost-lte-pie-t3852725

Got no response at first, and then I saw some similar threads with same issue but no resolution. So I started looking for myself. Lasted 2 days looking for a solution as I was completely sure it was not a Hardware issue as I had it before. So, I will explain how I get it back after combining a tutorial with some personal knowledge.

I need to give credits to: [MENTION=8638808]kirtikhurana[/MENTION], he does not even know I used his tutorial for Lenovo P2 haha, but it is worth it thanking him as his thread helped me a lot.

What we need:

QPST

QXDM (You can find this two programs on this thread, 3rd post https://xdaforums.com/lenovo-p2/how-to/enabled-4g-ca-p2-t3707584) Once again, thanks to the creator of the thread for this.

Minimal ADB and Fastboot

LTE Band Calulator app(Which you can find on play store)

Network signal guru app (Which you can find on play store too)

Stock rom (Then you can change it and flash whatever rom you want)

First thing, is have the stock rom rooted, with magisk or whatever method you choose to root it. We will connect the phone to our PC and turn on the USB debugging. For the ADB and fastboot, let's make sure we open the CMD with administrator rights.

I will assume you had already installed QPST and QXDM. We will open a CMD where we have ADB and Fastboot and type:

adb shell

and then:

su

We will grant adb shell superuser rights on our phone. As soon as superuser rights are granted, we need to enter on DIAG mode, for that, we will type on the CMD:

setprop sys.usb.config rndis,diag,adb

On the original thread, it is stated that you can also dial *#*#717717#*#* to enter directly in diagnostic mode (Use this option if QPST does not detect your phone on following steps and reboot).

Now, we will go to QPST>bin folder and open QPST Config, with administrator rights. Check whether your phone is connected or not on diagnostic mode. I will clarify something here:

On the second tab of the QPST Config, you will see an option to add ports, by default, you can select COM1, which is usually not the port where we connect the USB. If that happens and does not detect your device, go to device administrator in the control panel, and make sure your android phone is detected as Diagnostic device, you can check that on USB and ports, where you will also find COM1 as another device. As soon as you make sure drivers and everything is ok, but still does not detect the phone, do the step previously mentioned of: *#*#717717#*#* and reboot.

Now, assuming that your phone is showing there, you will now go to QXDM>bin and open on the tools bar, options and click on communications: You will see your device and the type of model of the processor, in this case, should be MSM8986 or similar. Check below and you will see an option called target port, check on drop down list and choose the com port where you have your device. Now you can close that window.

Now, we will go to view and pick NV Browser, then look for the filter LTE. Then we will select the first option (if this and the other option if grayed out means it has not taken the connection of your phone).

On that first option, you will see two fields below. We will choose the first one and click on read. As soon as we do that, it will autopopulate a number, that number are the bands that you have enabled.

Let's go now to our phone and open LTE band calculator, and let's pick up the bands we need to have (if you write a random number on the app, it will show you which bands will be selected with that number).

Having that number, we will, in QXDM, double click on the input field and write the number that the app has given us and then click on write. As soon as we do that, we need to reboot the phone.

Now, when the phone is already rebooted, we will open network signal guru, provide corresponding permissions and now, click on the three dots at the top right corner and click on lock bands, select LTE. Now, you will see that the band that you were missing, for you to get LTE is there! So what you need to do is select it and click accept. Your phone will freeze for a bit and then reboot itself.

As soon as the phone is rebooted, you will see that you have your LTE back!!

This was the way I was able to get it.

I said it was not a rom, because if you flash stock rom again, with miFlash, you may loose LTE again and need to repeat the process as it happened to me. However, if you are on stock and decide to flash another rom, oreo or pie, you will not lose it.

Hopefully is clear enough for you guys who are facing this issue.

Regards