Prepaid Sim doesnt register at network

You phone may very well be the problem.  Not every phone will work on AT&T.  Check the imei

https://www.att.com/buy/byod/identify?devicetype=phone

If the IMEI check doesn't offer to send you a Sim card, but instead says your phone is not compatible, then you need to replace the phone

Hello tobiasherden, we're here to help you with your prepaid account.

If you purchased an unlocked device from another retailer, please note, they are not optimized to work on AT&T’s network. This is important to remember when purchasing a non-branded, unlocked device and why you see different model numbers.  

First, make sure your device is compatible with AT&T’s Network. If it is not on the list, it may not work on our network. In instances like this, you’ll need to get a compatible device. 

If your device is compatible but is not connecting, the following may resolve the issues:

• Program the data settings for your non-AT&T device
• Get a 4G SIM for your 4G device or a 5G SIM for your 5G device
• Download the latest software – If you are an AT&T Customer, software downloads are automatic. If you have a non-branded device, you will need to go directly to the manufacturer.
• Make sure 4G is enabled on your device. Some customers disable 4G.
• Try a couple of solutions written by some community members
  • Solution 1
  • Solution 2

Let us know if this helps.

Thank you for visiting AT&T Community Forums!

Carlton, AT&T Community Specialist 

Dear both, thank you for the efforts. 

Dear formerlyknownas, right, my primary phone's (Samsung A53 5G model, AT&T also sells the model) imei is not shown as compatible. But I tried an older iphone, which is shown as compatible but also same behavior if I insert the SIM card there. Also, the phone worked all the way with AT&T network for the registration of the sim card and adding the first 10 USD...

At ATTHELP, indeed strange, but A53 is not on the list, even if it is a brand new phone and you sell it. Are you differentiating between PREPAID Sim cards and regular contracts? 

I dont want to rebrand the phone with AT&T software, as I still want to use it back home (germany) with my original SIM card (which connects without any problems to tmobile network in the US).

I tried 5G or LTE/4G with and without VoLTE on... no change. Settings for accesspoint are (as per default) ATT NXTGENPHONE. 

If this helps: the sim card allows for emergency calls. 

Thanks again for help and possibly any other ideas...

Tobias

If you activated with the A53, the service was suspended for using an incompatible phone. 

To unsuspend, you need to have At&t correct the IMEI.  IPhone must be 6 or newer. 

Thank you. Indeed the reset (3rd AT&T shop, you need a manager onsite) of the card and reactivation in the old Iphone did the trick... So all hapy now. Thanks again!

PS @AT&T still odd, that you dont allow your own sim card to work on that phone while a Vodafone Germany Card perfectly works in the phone and choses AT&T automatically for roaming. 

We appreciate the response, @tobiasherden. 

We're glad to hear this has been resolved, and you're now able to use your device with our AT&T PREPAID service. Your opinion matters to us, and we'll be sure to pass your feedback along!

Thanks for reaching out to our AT&T Community Forums!

Sydne, AT&T Community Specialist