This was posted on the Verizon thread. I'm pretty sure the dialer code is a Verizon thing but you can try it out. Other than that, maybe flash modems? I'm assuming you've searched this thread. Anyways, Here it is:
The following was posted earlier by user RyaNov133.
For those of you who have gone into Settings > More Settings > Mobile Networks > Preferred Network Type and then selected "LTE (recommended) from the list without any selected, here is what you do to get your service back.
I had done this because, hey nothing was selected, might as well select it right? Afterwards I lost all service and continually got a "Mobile Network Not Available" message whenever I tried to place a call. To fix this, I did the following:
1. Open Phone app to the dialer
2. Type *#*#4636#*#*
3. A testing menu should appear
4. Tap "Phone Information"
5. Scroll down until you see "Set Preferred Network Type"
6. Tap the dropdown menu and select "LTE/CDMA auto (PRL)"
7. Enjoy having your service back!
Just remember not to touch any mobile network settings and you should be fine.
This happens because (I believe, correct me if I'm wrong) the ROM was made for AT&T phones running on a GSM network, therefore the mobile network settings are programmed to try and pick up a GSM network, while our phones can only receive a CDMA connection. When you selected LTE (recommended) from the Mobile Network settings, it was referring to a GSM network.