If it was flown with a DJI drone, they tend to use the wrong datum for elevation, and can be off by more than 50m vertically. In Pix4Dmapper, I correct this by editing the imagery input table (the one with the EXIF values). I don't use Drone2Map, but I hear it was based on the Pix4D engine, so it might have a similar interface at the beginning of the orthorectification process. You can use a single value to approximate height, or If you want to get more accurate, you could take one ground shot before takeoff, and subtract the AGL elevation of that ground shot (should be 0) from all subsequent values. I've found the approximation method works just fine for my purposes, though.