they cant find my phone in the system so they claim they cant unlock my phone. what can i do, who can actually help for carrier unlocking my iphone

I know that someone here can lead me the right direction but please do read this entire description of my frustrating situation before you copy and paste tmobiles carrier unlock policies.

I've already been to multiple locations and called customer service everyday and everybody just kinda breadcrumbs me and doesn't really help (my guess cause I'm not actually buying anything and I wholehearted believe that)

Anyways, i bought an iPhone 10 on a post-paid plan in 2018 and stopped paying for the phone halfway thru the 2 year contract and I never paid so they sold the balance to a third party collections agency (ERC). Yes I still have the original receipt and yes I paid the collections agency. And yes I'm 100% absolutely positive I bought the phone from tmobile and not some MVNO like "visible".

But this is where it gets tricky, I paid the debt to ERC and have the letter from ERC sayimg that i paid it, it also has a reference number on that but the 1 store that ive been to says thats not proof of payment, its just saying that it was paid (exactly what the dude told me, sounds heinously dumb right?). But before that, my account has been closed for years so we can't see info from just my old account number, apparently they can't find the iPhone in the system using the IMEI number, its like me and my phone were completely removed from the systems. now some business that I'm not involved with uses the number. So it's useless trying to use customer service on the phone because they can't pull up info due to the amount of time since it was closed, and their not very helpful anyways cause I've called like 20 times and only like 2 people were helpful and they were from the activation department so they couldn't really help me anyways. What can I do? I don't wanna pay a few hundred bucks to start service on this phone just to unlock it in like 2 weeks, and the "f"olks at the tmobile store say come back with proof of debt payment when i literally have proof with reference number....what do I do...call debt and collections department next?