Credits are not being used

When you add credits to your account, they can only apply to usage starting from current month, and going forward into future months. If you already have outstanding bills and charges, billing periods for those charges are already over (and the invoices are already created), then credits cannot apply to those past charges.

You'd need to pay for your outstanding charges separately, and then you would be able to use credits for any future usage.

UPD - if you have issues or difficulties paying for your past charges, I suggest opening a support case from your account (even if it's suspended, you can still open a support case) with the billing category. Ask support team which options you might have, maybe to pay those bills in parts (sort of a "payment plan"), or maybe to get a payment term extention.