If i’m understanding your issue correctly…
Like in regular FNC, you need to support them structurally with an “anchor” (Like that special vfx people would use for something to build off in the air)
For any “Fortnite Prop” actors there is an option in the details panel “Forcibly Structurally Supported”
Type “struc” in the filter to find it easy.
It only exists on FN Props though. You will not find this on regular Actors or Mesh Instances.
Normally a UEFN landscape will handle this too but you do not have one so there is nothing to pass the “Structure” on from if that makes sense.