Unable to access Apple Music despite MTN subscription

You will have to resolve the subscription through MTN since that is who you are paying. It is a very common problem and they have to link the subscription to your account before you are able to access it. That is not something Apple can do because they have no access to your MTN account to do that.

Review the information provided here. There are many reasons why that might not be possible, but don't know what happens when you try or if you are seeing any error message.

• Check what payment methods you can use with your Apple Account in your country or region.
• If you're using an accepted payment method but your Apple Account is set to a different country or region, change your country or region.
• If the Add button is grayed out, you might be in a Family Sharing group and you use purchase sharing. Only the family organizer can have a payment method on file. If you want to use your own payment methods, turn off purchase sharing, then add your own payment method.
• Double-check that your name, billing address, and other information is spelled correctly and matches what you have on file with your financial institution.
• Some payment methods require that you verify through your financial institution's app, text message, or other means. Contact your financial institution for help if you're unable to verify.

Add a payment method to your Apple Account - Apple Support