I would guess you will have to send it in for repair. There are a few things that can cause short range. The antenna in the camera, the wifi board in the camera, the wifi board or the antenna inside the ST10.
Give this a try. A bunch of us where having the same problem after the last ST10+ firmware update. Yuneec suggested to try this. Said you should do this after every update. Never heard of that before but it solved my problem and a few other peoples.
Turn Everything Off Bird and TX
Turn on the ST10+ but not the bird
After it initializes select Model. Select it at the bottom right of screen.
Touch and Hold the model. Menu appears. Touch & DELETE.
Now turn off ST10+
Turn on the Bird. After it is on, Tilt forward from the back end 45 degrees two times to enter Bind Mode. Back light will start flashing Orange.
Turn on ST10+ Select Flight Settings,at this time it will ask you to go to model select and create a new model. Once you have giving it a name click save and then Select Bind. Bind the Model and Bind the Camera
Tap Return Button at bottom of ST10+ a few times to get back to main screen. You should now see the camera come up on display.
From everything I've experienced and seen on YouTube, the max is around 1000', but it TOTALLY depends on where. In a field with no interference or trees, I got about 800'. Around my house with power lines and all, I get about 500-600'. Sucks. I need more too, and I don't know that I trust the hocus pocus antennas that are supposed to help.