I have not had any issues importing this in to FEATool, could you confirm that you have the latest version 1.11.1?
On another note, the mesh generator currently Gmsh seems is quite inefficient for geometries created from a lot of facets which this STL model has. It would be better to reduce the amount of facets, convert to IGES/STEP format, or alternatively one can build this kind of mesh programmatically/manually with for example the
gridrevolve function
grid = gridrevolve( circgrid(), 100, 10, 3, 2 );
plotgrid(grid)
The grid variable can then be imported in to the GUI. One can also create a geometry from the grid, for example
impexp_stl( 'test.stl', 'export', grid );
geom = impexp_stl( 'test.stl', 'import' );
figure, plotgeom(geom)
this has for example been done with this example using a similar coil geometry
https://www.featool.com/model-showcase/06_multiphysics_01_resistive_heating1