Here is the list of all the official and flashable GSM modems for the Samsung Galaxy S4. These modems are for the Galaxy S4 i9500 only and have to be flashed using ODIN app. To flash, put the phone in download mode and then connect it to the computer.
Then, fire up ODIN on the computer and untick Autoreboot.
DDUAMDK – Dev-Host or host mirror
DDUBMEA – Dev-Host or host mirror
DXUAMDK – Dev-Host or host mirror
DXUBMEA – Dev-Host or host mirror
JKUAMD9 – Dev-Host or host mirror
UBUAMDE – Dev-Host or host mirror
UBUAMDK – Dev-Host or host mirror
UBUBMF3 – Dev-Host or host mirror
VSUAMDE – Dev-Host or host mirror
XXUAMDE – Dev-Host or host mirror
XXUAMDK – Dev-Host or host mirror
XXUBMF3 – Dev-Host or host mirror
ZCUAMDF – Dev-Host or host mirror
If the baseband still does not change, put the phone in download mode again and flash the same modem one more time. This should do the trick on your Samsung device.