Unlocked yet still locked 5c? Making it work with T-Mobile? WHAT DO I DO???

So, my old iPhone got stolen, so I needed to buy a new one. My current carrier is T-Mobile, so I need to buy a new phone full priced instead of for $99 as it is with AT&T, Sprint, and Verizon. I thought that was a bit of a rip off, so instead, I bought a used iPhone 5c (32gb) on Amazon. It claimed to be unlocked, however I took it to the T-Mobile store to get a new SIM card, and when they put it in, it didn't work. The phone only said "no service." The person who was helping me sad it might be locked to AT&T.

I've already tried:

• Restoring on itunes
• Selecting T-Mobile as the choice carrier
• Setting up with and without backing up all my files from previous phone
• Crying and shouting at the phone
• Rebooting multiple times

PLEASE HELP ME