How to disable camel casing Elasticsearch field names in NEST?
ConnectionSettings.SetDefaultPropertyNameInferrer() is what you're looking for. This method accepts a function that takes a property name and applies a transformation to it. The function is then called on each of your properties before requests are sent to Elasticsearch.
If you want to keep your property names untouched, then you can do this:
settings.SetDefaultPropertyNameInferrer(p => p)
p => p here just simply being a function that takes a string (your property name) and returns the same string unmodified.