I have very similar issue but the only difference is my Nav was working perfectly before I got OTA update. I contacted FCA and they said the issue will be resolved in next update.
My suggestion is ...
1. (I am not sure if this helps) Move the vehicle to good Sprint reception area. Check you have full signal bars (three) and 3G mark. Try to call FCA assist from Uconnect system. This is just to check your Sierra Modem is really working. If you hear the dialing signal, it means working. Otherwise, you will see some error message even with full Sprint signal bars. Once everything working, try to update the software again with USB stick.
2. Since our audio is old anyway (almost 5 years old), consider upgrading to 2018 GC audio, (UAQ radio, not RA4). There are some web site who is selling it. Also, there are some thread below.