[Android Q&A, Help & Troubleshooting] enderwap_and:
With the help of Xombiex, i;ve tried many many times to find a proper combination for ATT phones. What i have found so far (btw, i paid even some money for some image from Europe). So, let's start:

- if you have a Sonim XP8 ATT unlocked and you flash and write also modemst1 modem st2 and fsg you are.... f@#ked. Why? because the best that you can achive is Android 8.1 with both sims working and apn thethring. this setup is ACHIVED ONLY by using SONIM 8.1 archive from xombiex (is actually an archive from russia with manipulated baseband). To go even deeper, actually, you need from that archive only the baseband (which i have the feeling that is patched - files that you need modem_a and modem_b). Whriting these partitions on any 8.1 image will achive network signal. If you use 8.1 image from Xombiex will achive also dual sim. BUT these modems files - version: MPSS.AT.2.3.c8-1.137255.1.138439.1-GEN-180420-1925 - will not allow you to upgrade to Android 10 (if you image is OTA enabled, then OTA is downloaded on phone and fails on install).

If you have have android 10 and modify the modem_a and modem_b with Xombiex version, then the phone will not start (EDL is the way to recover).

So, as conclusion - any other modem file is not going to show you signal. you will see maybe 2 sims but no signal. i have the funny feeling that somehow, when you overwrite the modemst1, st2 and fsg you lose the unlock. Since the lock requires the baseband to send the prompt to insert unlock code, then new baseband maybe cannot handle this (maybe was customized for ATT?).

Also with Xombiex modema and b there is a drawback in dual sim configuration: if SIM one is active in LTE/UMTS/GSM (europe), then SIM 2 will see ONLY GSM and UMTS but only US bands. In Europe, there are few operators that use GSM and UMTS 2G/3G bands).

also, THERE IS NO WAY BACK if you flashed modemst1 and modemst2. even QCN backup seems that is skipping this flag for unlock.

Now, if you get ahold of a image from EU for Global SONIM and you are on ATT proceed with caution. NEVERflash modemst1, mdemst2 and fsg! I am following some forum in 4pda - russia and i know that the guys found a way how to "homebrew" an image of Android 10 with "patched baseband" BUT they are not sharing it. Only with money and only in russia face2face. Aparently, there is a big business in Russia and they are having "exclusive" service to unlock dual sim on ATT phones with Android 10 (trust me, i tried even to buy it only for me, no way).

So, this is my experience with an ATT that i messed up big time. Currently i am having 8.1 with dual sim (living in Europe the second sim is.... sort of used due to band restriction).

Also, please note that i have tried the following:

- QCN restore from similar phone (btw, if somebody has an ATT phone unlocked with 8.1 - original, single sim, please share modemst1, modemst2, fsg, and full QCN backup...- if i will succede to upgrade to 10 i will share my findings here)

- for second sim band issue - i;ve tried *#....4636. code and select different combinations / bands, etc. Nothing worked.

- i even played with EFS explorer, QPST software programming different bands - no luck.

So, as a conclusion and hint for other that like to "poke around" - modem a and modem b from Xombiex russian immage seems to be patched but limited to Android 8.1. Dual Sim full functionality and APN enabled is controleld by Android itself and here it seems that the most clean version (of ATT ****) is still the russian version share by Xombiex.

Cheers mates and happy debugging. I am tired, so for now, i am going to enjoy my phone :)