unlocking iphone 15

I paid $540 to unlock an iPhone to give as a gift to my niece in another country. However, T-Mobile mistakenly unlocked a different phone on my family plan, which I currently use, instead of the phone I intended-even though I provided the correct IMEI number.

When my niece received the gift, she was unable to activate an eSIM on it. Not realizing the issue was due to the phone still being locked, she took it to various shops, assuming there was a different problem. Eventually, we discovered that the wrong phone had been unlocked, which is why she couldn't add a new SIM.

When I called T-Mobile customer service, they acknowledged that they made a mistake in unlocking the wrong phone. However, they informed me that more than two months had passed since the original request, so they now want to charge me extra to unlock the correct phone. When I asked to speak with a supervisor, they hung up on me. I've  been a T-Mobile customer for over 10 years, and this has been a horrible experience.How do I resolve this issue? I'm asked to pay an extra $300 dollars to unlock the phone now