weak transmission signal

Maybe someone could point me in the right direction I can only get out about 8000ft before I loose signal why is this iff the drone is supposed to go 4 miles I've put my phone in airplane mode after I set my home point and I also have the channel set to auto i do kind of live around mountains...

I have owned my DJI phantom 3 pro for almost a year now. I use it almost every day for shooting farm and ranch real estate. The transmission signal has been so unstable it is extremely frustrating. Today I was shooting a development and I couldn't get more than one second of stable...