Creating custom Leaderboard using Google Play Games plugin for Unity

I am trying to create a custom Leaderboard, instead of just showing the Google Play one.
The authentication used in the Google Play Games plugin for Unity works when I only use the standard leaderboard from Google.
What I am trying, is to use the Google Web API (Scores: list  |  Serviços relacionados a jogos do Google Play  |  Google for Developers) to get the values I need.
I have used the WWW class in Unity to try to achieve this. The WWW class works for GET requests on for example, but when I use it on the google Web API url, it returns a weird error.
If I run my program in the editor, I get the expected 401 error.
But when I run the program on my Android device, I get “Java.IO.FileNotFoundException” instead.
I have tried the following URL’s

substituting “leaderboardId”, “playerId”, “timeSpan” and “collection” with the actual values.
All these calls return the “Java.IO.FileNotFoundException”.

I am guessing it has something to do with authentication, but I am unsure on what exactly is happening.

I hope I’ve explained the issue well enough, otherwise please feel free to ask for a more detailed explanation.

Thank you.

we finally got success for adding custom leaderboard in our game Up : Adventure time

but it is showing limited number of users from global leaderboard. we have now more than 5000 users but when try to fetch data for 20 users its showing only 4-5 users.