You may do better by raising this question on some DJI forums.
Other than maybe having bought a bad GPS...
If you have any transmitter (FPV, etc.) on board the quad, turn it off to see if that helps. Also try in a place that doesn't have obvious sources of RF (nearby radio stations, military, power lines).
GPS signal will be worse where it can't see the whole sky, like between buildings or in a canyon, or indoors.
Your 5V power may be noisy.