[Galaxy S III Original Android Development] NewbieToo:
Temporary (BAD) Cell Data Fix for i9300 without LTE

First, thanks again for all the support, including the EFS info. In addition to temporarily thinking I solved the Cell Data issue, as described below, I've also backed up my EFS using the Partition App you recommended.

The ArchiDroid2 rom that was working on my S3, including Cell Data was downloaded from: https://xdaforums.com/galaxy-s3/development/rom-archidroid-v2-4-6-power-hands-t2354859. However, there have been some MTP crashes with this as well. And following the testing completed below, I re-flashed ArchiDroid2 to check its APN settings, and it went into a bootloop on me as well, so who knows at this point...???

My i9300 Info, based on Sticker under battery is: Modelo: GT-i9300, SSN: i9300GSMH, FCC ID: A3LGTI9300A. Motherboard has not been changed.

I was temporarily successful getting Cell Data to work, with certain caveats, using the T-Mobile link you provided (https://support.t-mobile.com/docs/DOC-2090) to set up a new APN. T-Mobile's instructions couldn't be followed completely. All other APN’s couldn't be deleted, as T-Mobile instructed, or the new APN would not work at all. Likewise, the other APN’s that the phone automatically defaulted to with this fix would not work either, unless the new Custom T-Mobile APN was created. In my own case, I deleted all APN’s with radio buttons, except “Brightspot (multibrand)” and the new “T-Mobile (epc.tmobile.com)” APN I created. The phone then defaulted to the Brightspot (multibrand) APN, causing Cell Data to work briefly, before the MTP crashed, Cell Data stopped again, and the phone eventually crashed as well.

The Temporary Cell Data fix was completed after flashing the following in TWRP recovery onto the i9300's newly formatted internal drive.

1. LindeageOS 14.1 for i9300, from https://mirrorbits.lineageos.org/full/i9300/20180110/lineage-14.1-20180110-nightly-i9300-signed.zip

2. GAPPS ARM 7.1 Pico, from http://opengapps.org/

3. Lineage Root using Addonsu 14.1 ARM from http://www.lineageosrom.com/2017/01/download-supersuzp-and-su-removalzip.html

4. Curiousrom’s Modem from https://xdaforums.com/showpost.php?p=75080878&postcount=3912

The final APN set-up was determined through the following actions:

Note: When setting up the Custom T-Mobile APN, set the following fields as follows, or the APN won’t work at all.

APN: epc.tmobile.com (for non-LTE devices

MMS protocol: This field was missing, so nothing entered.

APN Type: Internet+MMS

APN Protocol: IPv4/IPv6 (radio button)

Once the new APN is saved, T-Mobile suggests deleting all remaining APN’s, which I did not.

Instead, I deleted the remaining T-Mobile APN options available and rebooted, as instructed.

After reboot, cell data worked, HOWEVER, the APN automatically chosen by the phone was “Net10 (wap.tracfone)”. Cell data remained active when switching the APN back to T-Mobile (epc.tmobile.com).

After deleting the APN, “Net10 (wap.tracfone)” and rebooting the phone again, cell data still worked, but the phone automatically selected I-Wireless (I1.wireless.com) as the APN.

I re-selected the custom T-Mobile APN I’d created, then turned Cellular Data Off, then On again. The phone defaulted back to I-Wireless, so I deleted this APN as well.

I re-selected the custom T-Mobile APN I’d created, then turned Cellular Data Off, then On again. The phone defaulted to GIV Mobile (wholesale), then PTel, then Red Pocket, etc. etc., etc., all of which I deleted, one by one.

After deleting all remaining APN’s that possessed radio buttons, except one called Brightspot (multibrand), cell data still worked, but defaulted to Brightspot, even after resetting it to T-Mobile (epc.tmoblie.com).

Keeping only T-Mobile (epc.tmobile.com), I deleted the other last remaining APN with a radio button, “Brightspot (multibrand)”. The T-Mobile MMS (TMUS) and Ting MMS (wholesale) APN’s that did not have radio buttons were kept as well.. After rebooting, this resulted in LOSING Cell Data after reboot.

I then reset the APN’s back to default in order to regain all the original APN’s and start again. I suppose I could have left them all, and the phone would simply have defaulted to the “Net10 (wap.tracfone)” APN, but I chose to delete all but the T-Mobile and Brightspot APN’s. I also kept the T-Mobile MMS (TMUS) and Ting MMS (wholesale) APN’s, which did not have radio buttons. After rebooting, Cell Data worked, but it is Brightspot that is the APN chosen by the phone, not the T-Mobile APN I created.

Cell Data worked, but MTP soon crashed when doing a simple Google Search. All APN settings disappeared for a minute or so, then reappeared along with Cell Data. When I used the Lineage Browser, it crashed completely, unable to reboot beyond the Galaxy S3 Splash Screen. Reentered Recovery mode and wiped the Caches, but S3 stilled wouldn't boot past the Galaxy S3 splash screen.

Other Troubleshooting Attempts:

I restored the stock T-Mobile rom and checked the APN settings. You are right, in this rom, it is set to: epc.tmobile.com. This also results in an “H+” symbol above the cell data icon when at its best available.

Unfortunately, within LOS 14.1 APN settings, epc.tmobile.com is not available as an option. The only options for T-Mobile are: T-Mobile LTE (fast.t-mobile.com); StraightTalk T-Mobile (wap.tracfone), and T-Mobile IMS (ims). None of these were successful.

I set up a new APN, using the T-Mobile Link you provided, but that didn’t work either until the above settings were finally successful.

Just in case, I also tried to install the d2tmo nightly (“lineage-14.1-20171204-nightly-d2tmo-signed.zip”), but this resulted in the following error: “E:Error executing updater binary in zip ‘/externa_sd/…”