Are you in CE or FCC mode?
What are your surroundings?
Need more detail as to Phantom and Phone...
Is it that you cannot fly further than 300m? Or cannot get data to you phone past that point?
Help us help you!
Need more data!
Edit..o.k, I see av signal..still need more data!
Phantom with Wifi extender?
Try a different phone/tablet!
Probably a Vision, thats about the limit for FPV with the RE500, try buying the RE700 unit, its the range extender that comes with the Phantom 2 Vision Plus model.