Have you done any Mods, or is it all stock?
Before you launch the Dji app on your phone go to your wifi settings on your phone and see if it's finding the phantoms wifi 1st, I am going to guess it's not ,
Is your wifi transmitter flashing green on the left hand light ?
If that's flashing then the wifi transmitter is probably ok, then it could either be the phantom or your android.
You need to check this then report back.