The C5 transport would never fly to Ethiopia, for that would truly be an invasion of airspace.
The only way is to change planes in Djibouti. Of course, there are no commercial flights now, and if you want to fly to Mekelle, you have to find your own plane.
The solution is actually simple—pay a high price, a very, very high price to charter a plane. Just report a fake destination, then turn towards the airspace over Ethiopia. As long as you're not shot down by Airin Formation's fighter jets or anti-aircraft missiles, you're fine.
Problems that can be solved with money are not problems at all. It's the ones that money can't solve that are the real issues.
The biggest problem with landing in Djibouti is that Gao Guang's whereabouts would be exposed, because the stationed troops here are obviously colluding with the other side. Since Gao Guang's transport must land in Djibouti, the opponent will surely know as soon as they arrive.