The respective resource might not be accessed because the tileset is not in the view frustum of the camera (edit: although it should at least access the top-level tileset.json file. If this is not happening: Is there any output on the Developer Log of the Unreal Editor?). Double-clicking the tileset in the World Outliner should center the tileset in the view, and cause the requests for the tile data to be sent out.
But… the data indeed seems to be based on an old version. Specifically, the B3DM files of the tileset seem to contain glTF 1.0 data, because it prints error messages
LogCesium: Error: [2021-04-17 13:44:35.306] [error] [GltfContent.cpp:30] Failed to load binary glTF from https://vectortiles0.geo.admin.ch/3d-tiles/ch.swisstopo.swisstlm3d.3d/20201020/5/18/14.b3dm:
- Only binary glTF version 2 is supported, found version 1
It could be possible to upgrade the tileset, using the upgrade functionality from https://github.com/CesiumGS/3d-tiles-validator/tree/2.0-tools/tools (note that this specific branch has to be used), but there might still be an issue with the resulting glTF data, which is currently investigated.