To prevent against this, my suggestion is that everyone take and keep a backup. Here's how.
The EFS Partitions:
On the Mix 2S there are three partitions you should back up. modemst1, modemst2, and fsg.
modemst1 can be found at /dev/block/sdf6
modemst2 can be found at /dev/block/sdf7
fsg can be found at /dev/block/sde36
How to back them up
First, get into TWRP. TWRP and instructions for flashing it are here:
Once TWRP has loaded up, connect the USB cable from your phone to your computer, and, assuming you have installed ADB already, get an adb shell by going to command line and typing:
dd if=/dev/block/sdf6 of=/sdcard/sdf6.img dd if=/dev/block/sdf7 of=/sdcard/sdf7.img dd if=/dev/block/sde36 of=/sdcard/sde36.img exit
adb pull /sdcard/sdf6.img adb pull /sdcard/sdf7.img adb pull /sdcard/sde36.img
Research regarding this
For those interesting in research in this, I have found the following:
• If you wipe modemst1, modemst2, and fsg your IMEI and all NV data on the modem is lost, and the radio does not work.
• You can make a backup with QPST and restore it, and it will restore your IMEI numbers, but the radio still will not work. This is something I'd like to get working very much.
• I am still trying to learn more about this, so if anyone would like to help, please send me backups of sdf6, sdf7, and sde36 of the Mi Mix 2S and let me know which spec you have. I think it may be possible to convert the limited LTE band version to the full LTE band version for example.