edit: I should mention this fixed issues with Sprint for me, but perhaps googling in this vein could lead to finding a similar fix for other carriers.
tl;dr: I booted to recovery (TWRP), wiped, flashed CM12.1, 5.1 GApps, and the
sprint.zip file from this XDA post and got LTE back.
I'm not positive my issue was the same, but the symptoms are very similar so I'm adding the solution I found. My LTE was working fine on CM12 and CM12.1 for perhaps 3+ months. My phone battery died overnight a couple days ago, and when I powered it on the next morning it didn't have LTE anymore.
Not only that, but I was getting the "APN settings not available for this user" under Settings -> Mobile Networks -> APN settings. After googling, and googling, and googling, I think I tried about every solution suggested for this issue:
- setting /data/data/com.android.providers/telephony/database/telephony.db to higher permissions (e.g. 761)
- deleting telephony.db and telephony.db-journal and letting the system re-create it (theoretically in the "proper" manner)
*#*#4636#*#* and playing with the preferred network (LTE/GSM/CDMA, LTE only, etc.)
- Turning on wifi, rebooting, waiting a while, and then turning it off and trying to connect to data
- Adding a custom APN, which often wouldn't even save if I had the "APN settings not available for this user", and didn't seem to do anything otherwise.
- I even went back to stock (upgrading radios and firmware in the process) by applying the latest RUU here. This updated my radios (I was on 3.31.651.2), and allowed me to update PRL/profile while I was at it. I didn't even have LTE from the stock ROM.
I ran into post where someone mentioned "the Sprint APN fix" so I googled for that and ran into this post, which points to this XDA post with the fix.
Downloading that file (
sprint.zip) worked great for me, despite that post being ~2 years old. I just did the following:
- boot into TWRP, wipe as usual as if installing a new ROM
- flash CM12.1 (I used the most recent M8 snapshot from 09-01-2015)
- flash 5.1 GApps
- flash the
- wipe cache/dalvik
Rebooted and had LTE right away during setup. Soooo relieved, as this was killing me. I'm still puzzled as to how it could have gotten goofed from a previously working setup, but at least it's behaving now. Hope this helps others.