So, when you go into the payment system, you can only see the 6 month and 1 month options and Premier Membership (12 month) is not visible to you?
This means that your account has an excess amount of advance membership already paid for. Jagex won't allow you to add another 12 months to your accounts already paid for membership.
The only way to fix this is to message Billing Support and get them to transfer some of your excess advance membership across to another of your accounts.
To get to a message form, go to the
Refunds
support page and use the
'Request a refund from Jagex'
hyperlink.
While a refund isn't exactly the issue, that is the contact point you need to use.