Ad Manager small publisher edition - Ad types & backup ads issue

Hi all, we have a seat in ad manager linked to adx to handle advertising inventory from some publishers and the issue we are having is the option **Ad types & backup ads (**that is under Inventory---> AdExchange rules) is not working. The idea with this option is if Adx doesnt have ads to show, instead of charging an adserver impression they would show a backfill URL as priority (every adx impression has no charge).

I spoke with ad manager/adx support and they are saying that the option doesnt work anymore (in my opinion so they can charge you if you exceed the "free" impression limit), when they moved Adx front end and integrated it into ad manager that function stopped working.

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Has this happened to someone and was able to solve it? in that case, how did you solve it?

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thanks