Question about custom kernel, NVRAM edits and OTA updates


Feb 20, 2020
Hello everyone!

I have a Mi 9 from China, which is running ROM 20.1.21 and is rooted with Magisk 20.3 and I have TWRP 3.3.1-46 as custom recovery.
My device does not have Band 20 unlocked. My carrier uses Band 20 for LTE, so I would like to have this band on my device as well.
My girlfriend has a Mi 9 from the Netherlands (where we live), which does have Band 20, so it should be possible to unlock band 20 on my phone, since the Snapdragon 855 does support this band.

I read somewhere (here) that the bands are locked somewhere deep in the system and that it is possible to unlock them by changing values in the NVRAM.
There is a very big chance I'm going to need a custom kernel to be able to write to the NVRAM, because apparantly Qualcomm added some anti temper software to prevent unlocking extra bands (and other features).
I also read somewhere on this forum (here) that every OTA update from ROMS installs a new original kernel.

With this information, I have the following questions:
- Do I need a custom kernel to edit the NVRAM to unlock band 20?
- If I do need a custom kernel, which one whould you advise?
- If I have a custom kernel, can I still update my ROM through OTA?
- If I can update through OTA, will the changed settings to the NVRAM remain there, or will the newly installed kernel also reset the values (and I will lose Band 20)?

I would really appreciate your help!