Yikes, helpmeplox! What you're going through sounds awful and we want to make sure you're being helped. Did you have an EIP that billed out in error? Or was the phone ordered at full price when it should have been placed on installments? Since an installment plan is a financing agreement that requires legal documentation at the time of sale, we don't have a legal way to set one up after the fact. But if it was closed in error, there are ways that we can typically restore the plan - though there's a small window of time where that's systematically possible. Either way, we need to be working to make this right for you.
Since the Support Community is a user forum, we don't have account access to take a look at the particulars of what's happening and determine what needs to happen next to resolve this. I understand you're looking for an address for written correspondence to provide feedback - and you can find that here, on our Contact Us page. However, I'd implore you to give our T-Force team a shot before you do - mail takes a long time, and depending on your situation we may be able to fix this for you now, but not later. It sounds like you've already put a lot of effort into reaching out to fix this error, but T-Force are easy to communicate with - you can contact them via DM or PM on Twitter or Facebook. You can even provide them the link to this thread so that they can see what you've already typed out here. Thank you for reaching out here - I hope you'll give us another chance to get this solved for you.