Needing to unlock a phone to go to T-Mobile

I've recently decided to leave Sprint because the towers in the areas I frequent simply do not work despite having full bars of LTE. After chatting with several customer service people who claim the towers are "fine" (with no way to chat with a local person who knows the area and send someone out there)and people in the store who claim "work is being done" in the area (for 3 months?), I've decided to leave Sprint for T-mobile.

T-mobile says my phone has to be unlocked by Sprint and the people in the store told me to contact customer service for that. So I chatted with a CA yesterday who assured me that the device was now unlocked. However, the device still doesn't work when I insert a different SIM card. I don't know how to accomplish this since the local people pass it off to CS who just tells me its fine and it should be unlocked. I have an iphone XR so it's not a GSM/CDMA thing. Is there a way to contact a higher level of customer service?