My payment method changed to invoice

[Holkil]

I recently took out a mobile contract for an Apple MacBook at £26 a month. I updated direct debit details and then on the 2nd Jan it said payment method has changed to invoice and then another email to say payment is outstanding. The mobile bill is not showing on my sky app so I can't pay manually and when I've tried to link accounts it's asking for last 6 digits of account number but then it's saying not recognised when I put it in. I need help to resolve issue and re set up direct debit. My broadband and sky glass is unaffected. 

[caesarome]

@Holkil 

You will either have to call Sky or send them a message via one of their social media links that are at the bottom of the page to get help with this.