[One (M7) Q&A, Help & Troubleshooting] cwhicks:
Writing this because it was 2 days of pulling my hair out to get LTE working. There may be shortcuts to some of this, I don't know them, or care to. I am writing this to help others. Currently I am on build 12-2015-04-08-Nightly-m7spr.

Problem: Could not get 3G or LTE to work on Ting's Sprint network. I would get the bars with an exclamation mark. Hitting the Reset Defaults as suggested elsewhere would sometimes bring up the list of apns, mostly there was an empty apn list. When they did come up, Ting was at the bottom, but there was no radio button to select it. You could also not add any APNs. If you tried and hit save, it immediately disappeared. (Hitting the reset would also sometimes get a temporary 3G connection that would be lost quickly.)

Solution: You need to edit the apns-config.xml that is located in the /system/etc folder on the local device storage. NOT the sdcard.

Steps: So to do this I used ES File Explorer to copy the apns-config.xml to the sdcard and then onto my desktop and used a text editor to go in and entered this line in the middle of the Sprint apn entries. I did this because Ting was at the bottom without a radio button on the original list and just hoped locating in the middle of the Sprint entries it would help.

The line I entered is below:

<apn carrier="Ting" mcc="310" mnc="120" apn="n.t8.ispsn" type="default,admin,fota,mms,supl,hipri,internet,dun" mmsc=*NOTE* mmsproxy=*NOTE2* mmsport="80" protocol="IPV4V6" roaming_protocol="IPV4V6" />

NOTE:The forum says I can't post a link so the mmsc entry in the line above needs to have the text *NOTE* replaced with double quotes around an h tee tee pee colon forward slash, forward slash mms dot plspictures with a dot com at the end. Hope that makes sense.

NOTE2: This should be replace with double quotes around six eight dot two eight dot three one dot seven.

This line was gleaned from several locations. The existing apns-config.xml and other websites with suggestions.

Now you need to get it back onto the device. You can't just do a copy onto the existing file because it is in use. So you have to boot into recovery. Obviously USB debugging needs to be turned on and I also turned on root access to ADB (don't know if required but I did it).

I use Clockwork Mod touch so once in recovery I hit Mount System to mount that partition so we can work with it. Then back on your PC, put your modified apns-conf.xml in the directory where your adb.exe is. If you installed HTC Sync, which has the required adb.exe, it is located in C:\Program Files (x86)\HTC\HTC Sync Manager\HTC Sync. Then on your PC on the command prompt type these commands.

adb push apns-conf.xml /system/etc

adb shell chmod 755 /system/etc/apns-conf.xml

adb reboot

After the phone restarts, go into settings, Modile Networks, and Access Points Names. You must now hit your three dots in the top right and Reset to Defaults. NOW your new ting entry should appear, WITH a radio button to select. On mine, it actually had already selected it. If not, select it yourself and exit out of settings. Tick tock, 30 seconds or more later, the delightful LTE appears in the bars. My long nightmare is over.

Quick Points:

1. You have to have a SIM card to get LTE. I don't think it does anything else on the HTC One M7 except LTE. If you bought a used phone (which if it is an M7 you had to have at this point) and it came with a SIM card in it is no good because it is registered to another number and can't be changed. Buy a new SIM from Ting, $12.

2. If you get Sprint - Invalid SIM with the new card, you need to call Ting and have them associate your phone number with the new SIM card. Mine was not. If you get this error, this has to be done before any of the steps above.