[mmahal]

I could not really make out what your SERO Plan is but the regular unlimited data plan includes Nav.
Here's the fix for you Nav not working, Hard reset your device. And then put in your info that you signed up for TeleNav with initially
I was told to do this on a Touch Pro by a Sprint Tech Support Rep, and it worked like a charm after doing the hard reset, of course backup anything you don't want to lose.
I had taken it to the sprint store the employees there had no clue and even the repair store couldn't do anything.