From your description, it sounds like either the biller is not really Roku, or someone has your credit card number and used it to create a roku account. In either case, I'd just report the fraudulent charge to the bank. They'll remove it, and will probably give you a new number so that whoever has your old one can't keep using it.
For further assistance with account and billing-related issues, please reach out to our support team directly here for your 'Account or billing' issue, here: https://support.roku.com/contactus. They will be able to further assist you.
had 2 recurring charges that roku refused to determine why unknown email addresses were using my card to charge for services. Rather than fix it they required me to get the cc co involved who had me issue a fraud complaint, cancelled my card, refund the fraud charges, and make me give new numbers to all my recurring charges. Then next month when the they try to charge my account and get hit with the charge backs, they will have to contact the users to get the correct card anyway. After I sent them a dissatisfied comment and laid out the situation, THEN they said the would review it....too little too late.