We are currently trying to keep the heading of drone at a certain direction while it moves linearly. To do this, we added a DO_SET_ROI after our first waypoint, after takeoff. This method works with other platforms (i.e. Solo 3DR firmware) but does not seem to respond when using Pixhawk 2.1 on current and previous firmwares (AC 3.5.5 AR 3.2.1 & AC 3.4.6 AR 3.1.2 & AC 3.4.5). We have also tested DO_SET_ROI after every waypoint with no success in resolving the issue. Any ideas why this may be? Thank you in advance.