7. Remove your StraightTalk AT&T SIM from your iPhone
8. Insert the T-Mobile SIM
9. Tap Settings->General->Cellular->Cellular Data Network
10. if you don’t see Cellular Data Network, restart your iPhone with the T-Mobile SIM in your iPhone. Repeat #8
11. Remove the T-Mobile SIM card. Insert your AT&T SIM card.
12. The Cellular option will be grayed out. Wait until it is available. Tap Settings->General->Cellular->Cellular Data Network
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