What is the firmware on your ST16 and H 480? Is it at the latest from Yuneec?
Drone Firmware
Typhoon H STD v3.04 Gimbal: v1.25, Autopilot: v1.35, Camera: v3.2.34(A)
Controller Firmware
ST16 Controller STD v03.01.b30
I fly in the Blue Ridge Mountains of NC from time to time. I must admit though that I'm usually above 3,000 ft altitude when I take to the air and the 390' limitation is not an issue. I've not tried starting in a valley and trying to get above the surrounding mountains. That's just not good practice for me as I don't see myself having a lot of sats for good flying from the valley unless its quite a big valley.
AFAIK unless you have a 107 license you're restricted to 400' AGL which means from where you lifted off. As
@Ty Pilot stated you used to be able to unlock that by contacting Yuneec and presenting your 107 license.