Upgrade to icloud+

You can upgrade to iCloud+ here:

Settings > Apple ID > iCloud

Scroll down and find Manage your Subscription, tap in and choose the plan that suits you. You can choose between 50GB, 200GB or 2TB of storage space.

You can find more information about iCloud+ here:

Upgrade to iCloud+ - Apple Support

You should be able to add a normal Credit Card, try going to:

Settings > Apple ID > Media & Purchases > View Account > Manage Payments > Add Payment Method

Are you not offered to add a Credit Card here?

You can also add a payment method from a computer, or online see more here:

Add a payment method to your Apple ID - Apple Support