What is radio/modem?
Radio (or modem also called) is what controls the cellular/voice connection.
You can check your current version of radio under Settings->About device and look under “Baseband version”.
When do I need to update radio?
Most of the time, radios do not need to be updated. Also, when you do an OTA (over-the-air) update, your phone will automatically get the latest radio.
The only time you need to update radio manually is when you are on custom ROMs when your wifi is not working after installing ROM. If that’s the case, check your radio version and see if you have the latest radio. If not, you can try updating to the latest radio using ODIN and radio .tar file.
How to Install Galaxy S4 Radio/Modems!
Download the file ending in .tar for your Galaxy S4 model. Use ODIN to install in Download Mode. Instead of using PDA option, use the PHONE option for radio.
After installing a new radio, check your “Baseband version” under About->Settings.
If your radio hasn’t changed, try re-flashing with ODIN but with “Auto Reboot” unchecked. Then when you see “PASS!”, pull battery, put it back in, then hold down Volume Down, Home, and Power buttons together to get into Download mode. Then re-flash radio with ODIN. When done flashing twice, reboot your phone by pulling battery then back in. Now, the “Baseband version” should have changed.
Download Galaxy S4 Radio/Modems
Sprint Galaxy S4 SPH-L720 Radio (MF9) latest
Credits – XDA