This has been discussed quite a few times on the forums and the AnswerHub.
You will want to make sure that you’re lightmap UV islands vertices are on grid lines. (You can find more information here: http://worldofleveldesign.com/categories/cat_udk.php#lightmapping)
https://forums.unrealengine.com/showthread.php?8491-Baked-Lighting-Variation-Between-Static-Meshes
https://answers.unrealengine.com/questions/92819/lightmap-uv-alignment.html
You will also want to make sure that you do have a lightmass importance volume in your scene around your objects.
To even out the edges on the modular pieces you’ll want to adjust a couple of things in the World Settings:
Indirect Lighting Smoothness: 0.6 (something around this level)
Indirect Lighting Quality: 2
Environment color: Changing this slightly up away from Black will also help with some of the shadowing around the seams.
Give these things a shot and if you have any questions feel free to ask! 