Iphone 4 no service. Switched from AT&T to Tmobile?

My brother gave me a gift… An iphone 4.
He jailbroke it a long time ago because it use to be his.
Now I tried to switch from his service AT&T, which he isn't a part of now (because of jailbreak)
to Tmobile with a Sims card and activation kit at Walmart (50 bucks including the sim and activationness and 30 dollar montlhly.
So I got all that set up and my iphone still won't work!
Now what?

Att iphones will only work on tmobile if they are unlocked.
jailbreaking doesn't unlock em.
i can unlock it for $20

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