[Android Q&A, Help & Troubleshooting] ashes_a:
Wanted to see if this resolved the issue for anyone else.

APN for Sprint and RingPlus listed at bottom of post

Original provider: FreedomPop

New Provider: RingPlus

Custom Recovery: TWRP

[SPH-L720] CF-Auto-Root: http://xdaforums.com/showthread.php?t=2291827

RingPlus APN: https://bestmvno.com/apn-settings/ringplus-apn-settings/

Change APN App: https://play.google.com/store/apps/details?id=it.appando.app.changeApn&hl=en

Offline SIM APN Database: https://play.google.com/store/apps/details?id=com.SIMAPNCheck&hl=en

** Always make a backup. A full system and personal data backup. At the very least backup personal data just in case something goes wrong, so you still have your pics and vids. **

Started with FreedomPop ROM. Switch to RingPlus. Activated service and updated PRL. 4g and mms were working at this point. Flashed to cyanogenmod 13 and I could make calls but not use 4g. If it set it to 3g in Settings -> Cellular Network Settings -> Preferred Network Type it would work without issues.

I flashed back to my FreedomPop ROP and updated PRL and Profile. Rebooted. Flashed back to CM13. Still no 4g.

Next I used oden 3.09 to flash stock 5.0.1 Lollipop ROM L720PUGOK3. Took forever to download and had to flash 3 times before it completed. Once setup, I updated PRL and Profile again. Rooted and installed TWRP. Flashed back to CM13. Still no 4g.

Tried to select APN from APN list in settings expect there is no APN list settings anywhere.

Tried to manually edit APN list but was not able to find a great details on how to modify the APN file. Its an XML file and it seemed too risky to manually edit with the lack of a good guide.

Next I used Change APN app. No APN's were listed so I manually entered in RingPlus APN. No APN was added to the list but I rebooted the phone. Still no 4g.

Next I installed Offline SIM APN Database. Selected, Search APN settings for this SIM. It informed me I had two APN's on my device. Both named Sprint World with the same exact settings. I selected one. Then I selected the third button from the top named Go TO Android APN Settings. Again, the APN list was blank. I selected the three dots or options button and selected Reset To Default. It asked permission to reset to the default APN. Immediately the 4g symbol showed up. Data was now working on 4g. I rebooted and 4g was still working.

I don't know if all or just some of the steps caused it to work. I'm not to eager to replicate the situation. If anyone is having the 4g issue and they have tried everything, please try these steps and let me know if it worked. Honestly, I think just doing the last step of resetting the APN to default just might fix it. IDK.

I would have just flashed to stock but CM13 is cool and small.

**** RingPlus APN ****

--

APN Name	RingPlus

APN	n.r5.ispsn

Proxy

Port

Username	null

Password

Server

MMSC	http://mms.sprintpcs.com

MMS Proxy	68.28.31.7

MMS Port	80

MCC	310

MNC	120

Authentication Type	None

APN Type	default,mms,ia

APN Protocol	IPv4

APN Roaming Protocol

Bearer

MVNO Type

**** Spint APN ****

--

APN Name Sprint World

APN ninet.spcs

Proxy

Port

Username

Password

Server

MMSC http://mms.sprintpcs.com

MMS Proxy 68.28.31.7

MMS Port 80

MCC	310

MNC	120

Authentication Type

APN Type default,hipri,mms,supl,hipri,dun,admin

APN Protocol

APN Roaming Protocol

Bearer

MVNO Type