Why can't my phone connect to cell network data?

Question

I have a Nexus 6P running CyanogenMod 13.0-20160819-SNAPSHOT-ZNH5YAO0IN-angler based on Android 6.0.1. I used to be running a CM nightly build and my cell carrier is Ting. Here's what happened:

I was running a CM nightly build from March-ish for a while. A couple days ago, I upgraded to the latest CM nightly. Immediately after that my WiFi stopped being able to find/connect to networks, and a day or so after the upgrade my cell network data connection stopped working. (SMS and phone calls worked, however.)

I decided to switch to a CyanogenMod snapshot release, so I factory reset the phone and flashed CM and gapps (at the same time). I wen through the setup and lo and behold, the problem appeared fixed. However, when I restored all my stuff via oandbackup (including, I now realize, some system apps), the problem reappeared.

Thinking that there was somehow some system app with corrupted data that I'd now restored, I used oandbackup again (along with a TWRP backup) and factory reset again. I went through the setup, planning to simply restore apps one by one until I found the culprit, but the freshly-setup install had the same problem. Therefore I restored the TWRP backup, which brings us to now.

So, why can't I connect to data? Most of the time this problem causes the cell indicator in the status bar to have a "!" however sometimes it shows just the bars, without a "!" or the "LTE" indicator. I believe this behavior occurs when the phone is first booted and goes away for the remainder of the boot when Airplane Mode is toggled, but I can't confirm for sure.

Here's what hasn't worked:

Wiping Dalvik cache

Trying most of the APNs, toggling Airplane Mode in-between

Restoring the default APN settings

Selecting a network operator (there used to be two or so "Ting" entries available; I only tried one - now nothing shows up at all)

Selecting "choose automatically" in the Network operators settings

Toggling the preferred network type to 3G and back to LTE (toggling to 3G caused everything but the status indicator bars [i.e. the "!" and "LTE"] to go to away; I suspect Ting simply doesn't have 3G available anymore)

Turning on Data Roaming and toggling Airplane Mode

Downloading a factory image from Google and running flash-all.sh (i.e. flashing stock recovery, system, radio, etc.)

ADB logs of me booting my phone to the launcher and waiting a couple seconds for it to connect to the cell network: https://gist.github.com/strugee/cd61dbcc461bb17216c360516e711d29

— strugee | Sources

Answer

Turns out this wasn't a problem with the phone. I called Ting customer support and apparently all that happened was I hit 5 GB of data usage, so they shut off my data automatically to prevent an unexpectedly huge bill at the end of the billing cycle.

According to the support person, you can self-service this by logging into Ting and looking at your device settings - the "can use data" option should be off. Turn it on and you'll be good to go.

— strugee | Sources

Why can't my phone connect to cell network data?

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