You can use one BIT per digit so you can easily found the frequency of any given digit in a range. You can solve every query in O(D * log(N)) where D = 10.

You know how to do it with segment tree then you must be familiar with the logic. Then I guess problem is in the implementation part.
Here is my solution.

By the way if you have multiple queries which can be processed offline, this can also be done using square root decomposition (also referred to as Mo's algorithm).