Hi there , it is not a hardware issue , it is strictly software due to some kind of model based policy restrictions.
telecommunications support has been developed by Qualcomm so the source o hlos files have not been given in public yet .
But the Qualcomm hardware supports b20 and aggregated b3+b7 (LTE+)
I am just asking if there's a chance of getting a workaround on this ...
I use MIUI V184.108.40.206.NDDMIEK ROM, the method that I used has ten phases: (1) installation of TWRP (2) installation of Magisk for root (3) installation of NV_Calculator (4) theoretical calculation (decimal and hexadecimal) of factory values of Mi Max 2 for verification of NVRAM (5) theoretical calculation (decimal and hexadecimal) of new values including band B20 (6) put my Mi Max 2 in diagnostic mode (7) installation in PC of Qualcomm tools for access to NVRAM (8) checking, confirmation and replacement of Values in NVRAM (9) uninstall Magisk to remove root (10) reinstallation of ROM MIUI by fastboot method with option "save user data" to remove TWRP.
You can follow this video
as a reference but the LTE values in the NVRAM of Mi MAX 2 are different.
For B1+B3+B7+B8+B20+B28+B32+B38+B40 European LTE bands I put the value 689476993221 (decimal).