Basecamp doesn't display contour lines of Open Topo Map

I have the following problem:
I installed Open Topo Map in Basecamp.
I downloaded the map from here:
https://garmin.opentopomap.org/#italy
and installed it following the instructions given there.
The map is displayed in Basecamp, but the contour lines are not shown.
How can I do to display the contour lines?
Could you please help me?