Originally Posted by asbesto
Here it is the solution:
To make the GSM option available again in the "preferred network mode" menu, you need to remove the custom overlay which is replacing the original menu with the vendors own.
using a file manager (I use ES Explorer with root access enabled) navigate to root (/) then to "vendor" then to "overlay"...i.e. /vendor/overlay
Now remove the following APK (or cut and paste to another folder):
reboot the phone, "GSM only" option should be available again.
Also, if you want to have the original "LTE" indicator instead of the 4G indicator, remove SystemUI-Overlay-285.apk (from the same location)
Sony Xperia Z1 c6903 Black --> Rooted, Kingston 64 GB SDXC class 10, Roxfit nero black gel case, front Vikuiti DQC160 screen protector, 4.4.2 stock rom
LG GPad 8.3 Black --> Rooted, Transcend 32 GB SDXC class 10, VOIA black jelly case, front Vikuiti DQC160 screen protector, CM11 4.4.2 Nightly