Sorry but it is insane to fly it to 400m high and 3km out without even knowing how to fly the thing.
Learn all the basics first, read the manual 3 times at least, play with the simulator, understand the software, know the rules.
The mavic is a great drone in capable hands. For newbies it seems it can do everything on its own and just pressing RTH brings it home all the time.
Well, most mavics are lost or crashed during RTH! It supposed to be a failsafe, not a standard routine you should use always. Unless you totally understand how it works and how obstacle avoidance and positioning sensors can interfere, and how you can take back control, even without app screen.