As DarthViper suggested you’ll want to adjust the lightmaps resolution and/or the UV mapping.
Since you’ve tried upping the lightmap resolution and are seeing no change then you would need to start getting a good lightmap UV in place. 512 for a wall is a bit high when you can get away with lower with a good lightmap UV.
Until Eric and I get a troubleshooting guide up on the Wiki for this part of our lighting guide you can check out this post which will cover just about all lightmapping issues with modular pieces and seams: Modular Asset Lighting Problem - Rendering - Epic Developer Community Forums