Just because you have 2500 rating, you are staying in Silver 2 since you don't have enough Collection Power to move up any higher. As a result, you continue to match only with players in Silver 2, and possibly some in Silver 1. Your DEC won't ever increase either until you move up in the higher tiers, same with the number of daily quest rewards you earn.
You have two options:
- Buy enough cards to increase your Collection Power for Gold 1 or Gold 2.
OR
- Rent enough cards to increase your Collection Power for Gold 1 or Gold 2.