Here is what I have found about the 3G data bug that some users get...
I compared radio logs from CM and MIUI and seems like MIUI telephony framework supports DC-HSPAP radio technology which CM doesn't and users don't get data connection. Here are the logs:

=========================CM begin================================
11-08 15:10:58.436 29978 29978 D RILJ : [0852]> OPERATOR [SUB1]
11-08 15:10:58.437 29978 29978 D RILJ : [0853]> DATA_REGISTRATION_STATE [SUB1]
11-08 15:10:58.438 29978 29978 D RILJ : [0854]> VOICE_REGISTRATION_STATE [SUB1]
11-08 15:10:58.440 29978 29978 D RILJ : [0855]> QUERY_NETWORK_SELECTION_MODE [SUB1]
11-08 15:10:58.453 29978 30064 D RILJ : [0852]< OPERATOR {MegaFon, MegaFon, 25002} [SUB1]
11-08 15:10:58.456 29978 30064 D RILJ : [0853]< DATA_REGISTRATION_STATE {1, 1e35, 00ae1683, 20, null, 20, null, null, null, null, null} [SUB1]
11-08 15:10:58.456 29978 29978 D PHONE : [ServiceState] setDataRegState=0
11-08 15:10:58.456 29978 29978 D PHONE : [ServiceState] setDataRadioTechnology=20
11-08 15:10:58.456 29978 29978 D GsmSST : [GsmSST] handlPollStateResultMessage: GsmSST setDataRegState=0 regState=1 dataRadioTechnology=20
11-08 15:10:58.458 29978 30064 D RILJ : [0854]< VOICE_REGISTRATION_STATE {1, 1e35, 00ae1683, 3, null, null, null, 0, null, null, 1, null, null, 0, d5} [SUB1]
11-08 15:10:58.458 29978 29978 D PHONE : [ServiceState] setVoiceRegState=0
11-08 15:10:58.458 29978 29978 E PHONE : [ServiceState] setState deprecated use setVoiceRegState()
11-08 15:10:58.459 29978 30064 D RILJ : [0855]< QUERY_NETWORK_SELECTION_MODE {0} [SUB1]
11-08 15:10:58.462 29978 29978 W PHONE : Unexpected radioTechnology=20
11-08 15:10:58.462 29978 29978 D GsmSST : [GsmSST] Poll ServiceState done: oldSS=[0 0 voice home data home MegaFon MegaFon 25002 MegaFon MegaFon 25002 UMTS HSDPA CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false] newSS=[0 0 voice home data home MegaFon MegaFon 25002 MegaFon MegaFon 25002 UMTS Unexpected CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false] oldMaxDataCalls=20 mNewMaxDataCalls=20 oldReasonDataDenied=-1 mNewReasonDataDenied=-1
=====================CM end=====================================

======================MIUI begin=================================
11-12 12:43:54.761 3482 3482 D RILJ : [4047]> OPERATOR [SUB1]
11-12 12:43:54.762 3482 3482 D RILJ : [4048]> DATA_REGISTRATION_STATE [SUB1]
11-12 12:43:54.764 3482 3482 D RILJ : [4049]> VOICE_REGISTRATION_STATE [SUB1]
11-12 12:43:54.765 3482 3482 D RILJ : [4050]> QUERY_NETWORK_SELECTION_MODE [SUB1]
11-12 12:43:54.771 3482 3960 D RILJ : [4047]< OPERATOR {MegaFon, MegaFon, 25002} [SUB1]
11-12 12:43:54.775 3482 3960 D RILJ : [4048]< DATA_REGISTRATION_STATE {1, 1e30, 009cecd8, 20, null, 20, null, null, null, null, null} [SUB1]
11-12 12:43:54.776 3482 3482 E GsmSST[1]: [GsmSST] error parsing GprsRegistrationState: java.lang.NumberFormatException: Invalid int: "null"
11-12 12:43:54.776 3482 3482 D PHONE : [ServiceState] setDataRegState=0
11-12 12:43:54.776 3482 3482 D PHONE : [ServiceState] setDataRadioTechnology=20
11-12 12:43:54.776 3482 3482 D GsmSST[1]: [GsmSST] handlPollStateResultMessage: GsmSST setDataRegState=0 regState=1 dataRadioTechnology=20
11-12 12:43:54.776 3482 3960 D RILJ : [4050]< QUERY_NETWORK_SELECTION_MODE {0} [SUB1]
11-12 12:43:54.777 3482 3960 D RILJ : [4049]< VOICE_REGISTRATION_STATE {1, 1e30, 009cecd8, 3, null, null, null, 0, null, null, 1, null, null, 0, 1c1} [SUB1]
11-12 12:43:54.778 3482 3482 D PHONE : [ServiceState] setVoiceRegState=0
11-12 12:43:54.778 3482 3482 E PHONE : [ServiceState] setState deprecated use setVoiceRegState()
11-12 12:43:54.779 3482 3482 D GsmSST[1]:[GsmSST] Poll ServiceState done: oldSS=[0 0 voice home data home MegaFon MegaFon 25002 MegaFon MegaFon 25002 UMTS HSDPA CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false] newSS=[0 0 voice home data home MegaFon MegaFon 25002 MegaFon MegaFon 25002 UMTS DC_HSPAP CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false] oldMaxDataCalls=20 mNewMaxDataCalls=20 oldReasonDataDenied=-1 mNewReasonDataDenied=-1
========================MIUI end==================================

As you can see CM gets “Unexpected radioTechnology=20” as the last one defined in framework is 19 and no data can be setup. But MIUI recognize it as DC_HSPAP and data works.

My research shows this is very rare situation with some devices. Not a lot in CM Gerrit and nothing for CM13. DC-HSPAP was removed sinse CM12!? Have seen some support added to old Cms like https://review.cyanogenmod.org/#/c/73163/ and https://review.cyanogenmod.org/#/c/74931/ but nothing for CM13. Don't even know if this going to work... Gerrit search for DCHSPAP https://review.cyanogenmod.org/#/q/DCHSPAP

The strange thing is those old radio blobs from you 2nd release work but anything after that doesn't and yet they both have the same “Unexpected radioTechnology=20” errors in the radio log!?!?!? The logs are actually similar!?!?!?

I might be completely wrong...

If I get it init/init_hydrogen.cpp is where all the setprop for hydrogen/helium magic happens...
But sys.totalram returns same value for 3/32 hydrogen and 3/64 helium isn't? So not reliable for device identification.
What if you use read_file2 function to get soc_id from /sys/devices/soc0/soc_id. That shoud be reliable enough, always 266 for msm8956/hydrogen and 278 for msm8976/helium. Another option is /proc/cpuinfo to get cores count or something...
Just like init_alarm_boot_properties() function uses read_file2...

Not an expert so excuse my ignorance if that is not going to work...

Good to see you back working on Mi Max!
Wondering about the reason behind changing GPU from 510 to 405 in BoardConfig.mk
Thanks!

At xda we just discovered Mi Max has two different hw variations:

EMMC: Toshiba
DDR: Samsung
LCD: TIANMA FHD R63350 VIDEO PANEL
TOUCH IC: Focaltech
TP Maker: Ofilm

and

EMMC: Samsung
DDR: Elpida
LCD: AUO FHD R61322 VIDEO PANEL
TOUCH IC: Atmel
TP Maker: Biel D1

These needs to be accounted for for example for dt2w to work for both and other hw dependant features as welll. See http://forum.xda-developers.com/showpost.php?p=69473153&postcount=1423