I don't know if someone can help me, but I really don't know what to do with this problem.
I bought one G900M (version from Brazil) on the release date and after a few weeks I install a custom rom and the TWRP recovery.
The problem is: I tried to go back to the stock ROM, but I still have the recovery with the CUSTOM "mark".
I tried to re install many times, do a wipe/format from the recovery, with no success.
I always receive the message when I try update the phone by OTA what say "The operating system on your device has been modified in an unauthorized way. Try downloading...
I appreciate any help.