If you have made a one-off payment (in the amount of 5.50 euros), you can purchase your subscription at any time extend simple as:
Open the “Membership” page, select the Basic Membership and then the “5.50 euros for a month once (no subscription)” option in the “payment without subscription” area. At an existing Premium Membership only the paid period will be credited automatically once your existing premium membership expires.
When redeeming a voucher the renewal of a subscription is not possible. Since premium memberships that were paid by means of a voucher do not automatically renew, you have to wait before you redeem either a new voucher or pay a subscription until the expiry of the coupon period.