You rolled back to GB? or is there a way to just roll back to the EL29 modem?
You can just flash your modem using Odin. There's no issues at all doing it, as any modem version is compatible with any software base, even across different OS versions. I happen to actually be running the EL29 modem right now.
Fyi having a non-current modem on your phone will cause future OTA update installations to fail, so you'd need to flash back to the current modem before you take an OTA update.
If you're interested in flashing your modem let me know and I'll reply back with instructions once I can get to a PC so I can link some files.
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