Thanks to @phhusson for noticing why the Hardware Overlay Picker was faulty (and @Exelios for giving him the phone), the LTE overlay now gets enabled properly and it's working as intended in the Razer Phone.
With future compilations of GSIs from him, this issue will be solved.
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However, if you do require to solve this in your current set up, just like I did, get the /system_root/system/priv-app/HardwareOverlayPicker apk, decompile it, open the AndroidManifest.xml and add this right after the package=".........." part:
Now recompile the apk and download a copy of SignAPK and sign the apk with the platform keys (platform.*) from Android AOSP.
You can do so by using the following command:
After that, push it to your system_root/system/priv-app/HardwareOverlayPicker folder, delete the old apk if needed and reboot. You should be able to choose LTE now as network mode.
Or... If you're too lazy, use this version I modded (extracted from a GSI from the 26-03-2018, so be careful since it's a bit old).
BE WARNED, NONE OF THESE STEPS ABOVE IS NEEDED IN UPCOMING VERSIONS SINCE IT'S ALREADY FIXED!!!
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Enjoy your bug-free
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