What happens when you try?
If the cash credit on your account is at least as much as the cost of the plan then you should see the option to pay using the cash credit instead of your bank card.
Are you seeing that?
What happens when you select it?
Any error messages?
If the cash credit on your account is less than the cost of your plan then you won't get any option to (part) pay for the plan using it.
If you need to contact 48 support about this then see the links in my signature below.
