BLU unlocked tablet.....

I paid for pre paid  $30 monthly 3GB data plan... 

My unlocked BLU tablet reads the sim recognizes everything but I don't get any internet connection.... 

I don't know if its APN or if its another kind of problem.

I put another sim from a contract Att phone let's makes call and receives calls but still I cannot access internet.

Any help would be great thanks