I suggest trying dynamic lighting for one or the other, or both, skylight and directional. But if you want to remain using static / stationary for exterior and ext-int combo, then delve into how to create materials that are properly created for it. Stationary lighting has a number of bugs or errors often seen in exterior scenes and other scenes where exterior lighting heavily influences interior lighting. Light portals, lightmass importance volume, the volumetric lightmap, and distance field methods are all a part of producing photorealistic results for exterior + interior scenes, and exterior only (except for light portals probably). Are you utilizing ray tracing in any scenes, intending to use it? If so, materials design is important for speed / quality (there’s the RayTracingQualitySwitchReplace node), yet it has limitations in the area of reflections and translucency sometimes. One of the issues with exterior or combination E/I scenes is the light bounces off the bottom hemisphere (with a skysphere present) and causes brightening of the undersides of geometry and with other wrong settings (such as Exposure) could result in overexposed / too much lighting in the scene overall. A setting that counteracts it is “Lower Hemisphere is Solid Color” and the color to black (other colors besides greyscale produce a transfer of the color to surfaces). Another thing that contributes to problems is incorrect World Lightmass settings, even for static / stationary lights only. Less bounces for the directional and skylight is actually capable of better results, and modifying post process volume settings (with one set to Unbound) is useful for adjusting the current settings of those two light types via Exposure, Global Illumination, and Lens (Toe, Shoulder, etc).
As far as downloaded models, what are the files’ types? FBX needs certain import settings to avoid artifacts, such as enabling ‘Convert Scene to Unreal’s coordinates’, ‘Force front x-axis’, and ‘Convert from FBX to Unreal units’. High Precision Tangent Basis and Full Precision UVs are two other settings to enable for meshes on Import or in the mesh-editor, along with the project setting “High Precision Normals” (GBuffer Format located under Rendering Project Settings). They’re suggested for quality reflections, but also can fix artifacts of imported meshes. Some models have better results when the normals are imported rather than created in Unreal, while others need to be created in Unreal, and tangents too. One thing to consider is how the coordinate systems differ between Unreal and external applications. Resolution of models from the external app is another one. If a model is at a high resolution coming from 3DSMax, and in Unreal it’s set at a default of a lower resolution, then it’s getting downsampled, and is likely losing accuracy / quality (similar to compression loss). So, try to ensure the resolution in the external app is medium level, then in Unreal increase it to get the results according to Unreal’s calculation of the model’s resolution and thus, its shading. I read somewhere that it’s usually best to get an upsample than it is to get a downsample.
A few settings to use when encountering pixelation and inaccuracies in lighting / shadowing:
Shadow Resolution (in the directional light, goes from 0 -8)
Shadow Slope Bias
Dynamic Inset Shadow (with stationary and movable lights)
Ensuring lightmaps are in the correct lighting channels (1 and 2, since 0 is usually for texture maps) and those lighting channels are enabled in the lights and meshes
Two-sided is enabled (or disabled, depending on the model / materials and its place in the scene) in materials, meshes, and for lighting and shadowing