[Lenovo Zuk Z2 Pro ROMs, Kernels, Recoveries, & Oth] tiggerle666:
BUG 1 - No 4G working, modem flash doesn't help (**SOLVED**)

The problem:

After properly installing the AOSP 10, I have no 4G access anymore for my pirmary SIM card (see post above, I flashed ZUI 1.9 and updated to ZUI 4.0 via OTA. I had 4G in all ZUI versions! Then installed AospExtended-v7.3-z2_row-20200612-2243-OFFICIAL after unlocking and flashing TWRP).

I am using 2 SIM cards from different providers for many years. After installing AOSP, one SIM card is provided with 4G access, the other one is never proposed to use 4G. The availability of 3G/4G seems to be linked to the SIM card itself and not to the position SIM1/SIM2 (I am aware that you can't have both cards be using 4G at the same time). It also affects my primary SIM card if I just use it alone. Besides, the "settings version" menu within mobile network is not accessable, don't know why...

With further testing I realized that the same issue with not providing 4G also comes up after installing AOSP 9. So all in all - this might be more a problem of the firmware, I guess? Does AOSP install new radio binaries or interferes with them? I do live in Europe - I mention that because of the known band 20 issue.

Trying to solve the problem on my own - how I failed:

After more than 60 hours of testing different approaches I haven't found any solution. In the following paragraphs I am listing up what I have tried out without success, just in case someone encounters a similar problem.

# Flashing /modem

I tried to flash /modem as described here https://xdaforums.com/t/radio-modem-basebands-for-zuk-z2-pro-only.3715835/page-12#post-84373123, but non of the files brought back 4G for both SIM cards. Not even the version I had sucessfully been using before with an older AOSP 8.1 was working now.

I tried all sorts of different combinations of flashing ROMs with incoherent results. I often bricked my phone and had to use the deep-flash cable. Somehow the ROM and the bootloader wouldn't match, I guess. Frankly, this even occured after doing a full backup restore in TWRP of my clean ZUI_4.0 state, which I used as a base to try out further changes in the binaries. So, sometimes after bricking my phone and starting again by flashing ZUI 1.9 there wouldn't be a 4G access at all afterwards, whereas in other cases the 4G access would come back just as it should. I couldn't find any consistancy there in this behavior

# Losing IMEI & losing baseband

Once I even lost the MEID/IMEI1/IMEI2 after erasing modemst1 & modemst2. It is not suprising that they are lost after erasing these partitions, because the IMEIs are said to be copied there for proper network usage. But it was surprising that modemst1 & modemst2 were reinitiated after erasing them (as they should), but the MEID/IMEIs had gone missing. It took me a long time to figure that out. If I checked for "baseband version" and "About phone" in the Android settings it was "unkown" and if I checked with *#06#, my MEID, IMEI1, IMEI2 were empty. I then even had come to the point were I rooted the phone with Magisk and tried to assign new IMEIs using Qualcomm_Smartphone_Write_IMEI_Tool_v1.01. Though establishing connection on the correct port, I always got a time out error, so never succeded there. Thankfully, flashing ZUI 1.9 a second (!!??) time brought back my IMEIs within the system.

# 4G usage within AOSP

In general, if I installed AOSP from ZUI_3.5 or 4.0, there would be NO 4G access at all in AOSP although it had been there in ZUI just the minute before (no matter if I wiped davlik/cache/system/data or not, I tried both).

The only way to get 4G within AOSP was to get to ZUI_4.0 and THEN flash again ONLY the according NON_HLOS.bin of that version (from the modem thread quoted above) to /modem. My interpretation is that this NON_HLOS.bin must in some way be different from the original one already being there on my phone, although it says it is from the exact same ZUI version. In this state, the following BUG2 didn't occur, it only occured if no new NON_HLOS.bin was flashed before installing AOSP. If Iflashed it after installing AOSP, it would result in a lost network connection. If I installed a radio_modem***.zip or even erased the modemst1 & 2 partitions, that resulted in NO network connectibility at all, meaning in the statusbar you'd read "no network" "only emergency call" and no network symbol would be displayed! This was due to the lost IMEIs as explained above.

THE SOLUTION

---> I was silly and wasted my time ...

Problem solved.

Over 60 hours of my life wasted. Although 4G and LTE are technically not identical, I wasn't aware of the fact that they are used synonymously and that it is hardcoded in the ROM whether you get "LTE" or "4G" proposed as an option in mobile network settings and accordingly get displayed as symbol in the status bar (although this was what I observed and suspected at some point, but never assumed that it would really be true).

If you want to know more about that, use google translate to read that interesting details here: https://en.droidwiki.org/wiki/4G_or_LTE:_What_is_shown_when?

To sum it all up, don't mess too much with your /modem partitions, haha ;-)

BUG 2 - Mobile data Network status faulty in system apps and status bar

There is an odd and faulty behavior of checking the network connection in Android. Note that this behavior does not occure if I had flashed the according NON_HLOS.bin version of the ZUI version already installed before I installed AOSP (see paragraph "# 4G usage within AOSP" above).

The system somehow believes that there is no internet connection even if there actually is, so Chrome would say "no internet connection" on top but you still can browse the net using 3G mobile data. And further, the symbols in the status bar go crazy:

Situation 1 - Mobile & WiFi data is switched off, symbol in statusbar shows no data network ("X")

Situation 2 - Mobile data is swichted on, symbol in statusbar still shows no mobile data network ("X") but also "LTE".

Situation 3 - If you activate WiFi the symbols in statusbar do now correctly show mobile data network

Situation 3 - If you deactive mobile data now, the symbols in statusbar do correctly show no data network ("X").

I'll try to report this to @dmd79 :)

Has anyone had the same problem with 4G and the modem drivers and found any solution??