Buying apps in Bangladesh

How can I buy apps on my iPad or iPhone 6.

Whenever I try to buy a apps from Apple Store, it shows "you must verify your payment info before you can make purchases."

Background Info:

I use these products here in Bangladesh. I always live in Bangladesh.

My younger brother bought these from Norway and presented me these apple products almost one year ago.

At that time, I have used an American address as my billing address, Card type is marked as none.

I did so because another Bangladeshi apple user had recommended me using the American address so that I can register the product successfully.

Recently, I tried to use a visa card here in Bangladesh but I failed to use it. Then I bought a redeem card of USD 25 at a cost of USD35. My apple ID credit balance is USD25 now.

Now, I am trying to buy apps from Apple store. but the same message comes back "you must verify your payment info before you can make purchases.

Is there anyone who could extend their support.