Hi,
You would need to turn the Real Time Capture box on the Skylight of the HDRi backdrop BP to False; this will instruct the editor to use the lighting coming from the HDRi cubemap image. The Skylight’s Source Type should be set to Specified Cubemap and the Cubemap section should be populated (see image).
Other lighting actors will keep working as usual but the skylight will no longer update automatically if you move large objects, delete walls, etc.
Alternatively, you would need to add a static mesh (floor) underneath it, add an Exponential Height Fog to the level and adjust the size of the cubemap. Make sure that the Lower Hemisphere Is Solid Color is set to False in the Skylight component.
Posting this as it may help someone out there 
[UE5.6]
