Hi. I want to share my experience with you. See, I live in Mexico and here the only available bands for LTE are B4 and B2, and it was nice to see this phone being compatible with B4, so I ordered mine as soon as it was available in black color. It arrived last wednsday, and what I found? That it was not able to connect to LTE, I though it was a configuration problem, but then I revised the manual included in the box and it clearly says it's only compatible with B1/B3/B7/B8/B20/B38/B40. Damn, I felt so bad, I made a little research googling and realized that there are going to be several versions of this phone and the LTE support is going to depend of the selling area. This was hard to accept, but also the key of a posible solution. This is supoused to be a phone sold worldwide, and if there are several versions depending of the selling area I guess is easier and cheaper for production to make exactly the same hardware rather than one for each selling area, so maybe the only constraint for LTE support could posibly be something related to software. And so it was. I wanted to try to modify the NVRAM values to force the SoC to accept B4. For this as you might know you need an unlocked bootloader (for root) and a rooted phone. So, first I unlocked the bootloader, something really easy to do by the way, and then looked for a CF-auto-root zip (it is officially supported by Chainfire), then flashed the boot.img aaaaand, when my phone turned on, IT WAS CONNECTED TO LTE SIGNAL, no NV values were changed, anything else but this was done to make Mi A1 to support LTE B4. I can't see which other bands were enabled, I don't know how to see this since signal guru is not free for this feature anymore, but I assume that also other official supported bands were enabled.
Hope this can help someone else in a similar situation than me. I don't know if flashing a boot partition is enough to change LTE support, if so, its probably that the phone locks again with an update, but I guess that reflashing the boot.img or even a full ROM can make it support LTE bands again, and the worse scenario would be to change NVRAM values cause I alredy confirm that its able to support more bands than stock, I mean, the hardware is there so you just have to enable it. And yes, LTE is working fine with no issues.