YOU HOLD MYSELF AND XDA FREE FROM ANY LIABILITY BY FLASHING THIS RADIO. DO SO AT YOUR OWN RISK. WE ACCEPT NO RESPONSIBILITY FOR WHAT YOU DO TO YOUR PHONE
IN ORDER TO ACCEPT ANY FUTURE UPDATES YOU WILL BE REQUIRED TO FLASH BACK THE STOCK 4.4 RADIO. FAILURE TO DO SO MAY LEAD TO A BRICKED DEVICE WHEN ATTEMPTING TO UPDATE.
I managed to get my hands on the 4.4.3 Radio for the Droid Ultra, Maxx, and Mini. I have attached two files below, one containing the files for the new 4.4.3 radio and one with the stock 4.4 19.6.3 radio. Download both.
1) Download both radio files.
2) Unzip the 4.4.3 files into the same folder in which you have adb setup. In my case this is c:/sdk/platform-tools
3) Open a command prompt and navigate to that directory
4) run the command
adb reboot bootloader
5) run the commands below to flash the 4.4.3 modem
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
5) When the phone is rebooted navigate to phone info and verify the new baseband version.
6) To return to stock 4.4 19.6.3 radio, simply remove the files you placed in platform-tools, and replace them with their 4.4 counter parts. Then repeat steps 1-5.
Been running the baseband while porting 4.4.3 and found no issues so far.
4.4 Radio 19.6.3
Donations are appreciated!
Droid Maxx and Droid Ultra
Both Unlocked and Rooted