Ultimately, does entry into Wakanda have to be by plane...
You can enter Wakanda in any way you want but the entry methods are protected.
Firstly, of course, you have to know that the super-secret Wakanda exists but let's assume that you do.
Then it's really hard to get to in the second place.
The Black Panther film further established that...it is a landlocked country in the central mountains far from the coasts. Impassable mountains and jungles around its borders have helped Wakanda isolate itself from outsiders.
If you DO know, and manage to pentrate to the interior then you have to cross the border itself...but the borders are patrolled and protected by The Border Tribe
The Border Tribe is a Wakandan tribe tasked with protecting the nation from foreign threats.
At that exact coordinate always?
Killmonger would not know the coordinates (unless his father told him and there is no indication of that) but still manages to fly into Wakanda.
It's apparently also worth noting that the majority of Wakanda is undeveloped and it appears that the major/capital city is the primary developed / advanced area. We see extensive jungle / farmscapes but very few apparent advancements in the outskirts of the country.