I noticed that I wasn't able to receive or send MMS or access mobile data with my phone using the CM11 ROM I had used for years. This was new, it used to work so I set out to fix it.
Step 0 (please skip to Step 1 if you don't like slamming your head against walls):
Have no data with CM11 that used to have data. Upgrade to CM12.1 after spending a couple hours figuring out how to do that because you haven't changed your OS in years. Find out data still doesn't work. Find out that CM12.1 does something weird where it won't show you APNs if there is a problem with the APN. Try all the fixes for this APN problem, one of which is rebooting your phone until it magically reappears. Go back to Stock, yay data, boo stock sucks. Install CM13. No data. Die inside. Search google for problems, learn about all of them. Too many solutions, none work. APNs. Why? Find a couple promising leads, none of which work. Decide you'll just stick with stock. Try it. Nope, can't do it. Nope nope nope. Day 2 now. Puttig together everything and following Danny Amborne lead gets you to Step 1.
What you'll need: MicroSD card, USB cable to connect your phone to PC, a fast internet connection or patience.
Tips: You'll need to know/learn how to use the command prompt for step 3. Putting all the downloads into the same folder will help once you reach the command prompt section, so do that.
Reset the phone to stock with this "1-click" (Only for SPH-L710)
At this point I was able to use data again to access the internet, and using the Samsung messenger I was able to send/receive MMSes. I was not able to send/receive MMSes with Google messenger though, not sure why.
(Tip: I had to install the USB drivers linked in the forum)
While I was here I did a carrier reset and updated my PRL and Profile as per ting's instructions at https://help.ting.com/hc/en-us/articles/205428288-Completing-Activation-for-your-Samsung-Device
Familiarize yourself with how to install CM12.1 using TWRP and download all programs
(Tip: I never needed the android SDK,I was able to put files on the SD card with the USB connection.
(Tip: don't follow the download link for TWRP labeled "download" use the "TWRP" link and download the latest version)
(Tip: if you get an error when using the cmd prompt with heimdall try installing one of the different drivers with zadig.exe)
(Tip: when you download the TWRP recovery file rename it "recovery.img" and you can copy and paste the command from the CM wiki into the command prompt.)
Download GAPPS (Google Apps) for CM12.1
Download Ting APNv5.zip (Danny Amborne's custom ting APNs)
Where he reported it worked with his S3 and he provides the link:
Direct link to Ting APNv5.zip
Put the CM12.1, GAPPS, and TingAPN .zip file into the root (base, first) directory of a microSD card in your phone. You can do this at any point. I was able to do it via my PC's usb cable connection even during TWRP recovery. I did not have to use ABD as explained in step 3.
I did a complete format of the internal storage at this point, just to be safe. You would need to do a wipe at least, if what I've read everywhere is to be believed. Install CM12.1 as you learned in step 3 (from the microSD) but DON'T REBOOT until you've subsequently installed GAPPS5.1.zip and Ting APNv5.zip. After that, reboot.
Set your phone back up. If you're like me you'll have data (yay!) but for some reason MMS will not work. Danny Amborne, you have failed me, just like all of the others. But…
Somewhere on the ting forums someone mentioned removing the proxy address from the mms proxy setting in the LTE APNs in the list found in settings>mobile networks>Access Point Names. I believe they were both the LTE *** named ones, but I checked all the APNs and removed the proxy address 126.96.36.199 from all 2 of them.
Test it out by sending a MMS. I read it can take a couple of minutes for changes in the APN to register, but I'm not sure. After this I was able to both use data to surf the world wide web an send and receive MMS.
This was a 12 hour ordeal. I wrote this up to remind me should I ever need to do it again, and to hopefully provide a specific resource to others with my same problem, as I had to incorporate many different posts in order to finally find a solution.