Its just way it is… the lines have no emissive light to prevent lighting inaccuracies and to save on performance.
Add a temporary point light with high intensity, or better yet a BP_skysphere with or without a directional light set this will light up any scene in seconds and just as easy to remove as it is to add.
Hope this helps
Don’t forget to accept an answer that best clears your question up or answers it so when the community finds your question in the future via search/google they know exactly what you did to fix it/get it going.