Ok, it’s what I thought it was.
Here’s ( the best I can do in a short time, excuse the levitating buildings ), the model on a piece of landscape
But if I try and put it on an HDRI backdrop
It doesn’t work. It’s because the lighting is all wrong. Basically you can use HDR for the horizon and sky, but never the ground, it will always look wrong.
Here, I put the landscape back, but keep the HDR for the rest
Sort of passable. I’m sure if I found an HDR of a desert it would be better.
So, nothing wrong with your model, just the lighting.
EDIT: I went and got a desert HDR
Meh, bit better. But… if you do this
More believable. I know I need to get the camera at head height, but I can’t because I don’t have a terrain that fits the buildings. All I did was use this great asset
https://www.fab.com/listings/273e2e99-03c5-4fac-a4c0-c7f6bf3c39f0
The lighting is better because we are lighting real objects. Does it make sense? 
With reference to your original pictures, the problems are
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No lighting and shadows on the HDR, also the scale of the terrain is way off compared the buildings
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Scale of the ground textures is just way different from the buildings. I think this one would work better with a believable sand on the ground.