Hello ,
Thank you for posting your issue un the community and it seems like the DXF is only containing 3D lines, and Twinmotion only reads 3D meshes information. As a workaround you could try to create a texture instead with the topo represented then apply the texture to the topo 3D mesh so it can be visible in Twinmotion.
You can also try and create 3D thickness/meshes out of your topo lines so they can be represented as objects/meshes.
Posting in the Facebook community will also get you some good other suggestions as there are more users there interacting with posts at this time. https://www.facebook.com/groups/twinmotion.community/
Kind regards,
Vincent B.