Cash credit on your account is never automatically used to buy a monthly membership plan.
If you want to use it to buy a membership plan then you need to do it manually.
If the cost of what you're buying is less than or equal to the amount of cash credit on your account then, at checkout time, you should get the option to pay by bank card or using your cash credit.
If you don't get the latter option then you'll probably have to contact 48 support for further assistance.
If you need to contact 48 support about this then see the links in my signature below.
