@Framac If it is showing in your account and you followed the 2 step process to purchase it. Can you manually select AT&T or the T~Mobile network?
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Have you enabled roaming on your phone? You may need to also reboot your phone before manually selecting at&t or T-Mobile.
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Getting US add-on is two part process, First top up your account balance and second buy the actual add-on.
If you did that and you can see that add-on in your plan details then reboot your device after manually choosing AT&T or T-mobile network.
If it is not your first time buying that add-on then wait for 24 hours to buy it again.