with iOS DD, use manual camera setting. Take off with DJI GO 4 to the height you set in DD, point camera down, activate AFC once focus switch it to MF then lock AE. Now switch to DD in mid air then start mission.
When ever I start a mission or after a battery swap, I always kill DD and DJI GO 4 then relaunch DD making sure its loaded fresh with no previous flight command. Then I take off with DJI GO 4 then switch to DD.
For new user, the sport mode in Mavic Pro is for you to override DD. That's how I normally land at the end of a mission.
I had some decent results with iOS DD with my Mavic Pro