Have you tried using the UV tool in UE4? It will get “correct” UVs for lightmass, though maybe not the most efficient for getting the best results. But is easier than doing manually if your not very experienced with UV unwrapping.
If you wanna learn UV unwrapping, then google some tutorials on the UV tools in your 3D app. And here’s a link to the UE documents on creating the best UV layouts for lightmass…
https://docs.unrealengine/latest/INT/Engine/Content/Types/StaticMeshes/LightmapUnwrapping/index.html