I cannot connect my mobile number as a payment method

Make sure this is one of the payment methods that you can use with your account, as listed in this support article: Payment methods that you can use with your Apple ID - Apple Support

If there's an issue when you set up mobile phone billing see this article

- If there's an issue when you set up mobile phone billing - Apple Support

If you still have questions or problems:

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support