Transferring a sim card from one phone to another removes the ability to use mobile data

Call at&t and correct the IMEI. 

Transferring a SIM card into a compatible phone would not disable mobile data. I suspect you are transferring your sim card into a phone that is not allowed on AT&T and will not work when 3-g sits down next February.

What is the make and model of the phone  you tested?

What make and model is your original phone? 

Hello @krappa, we'd be glad to help with getting your device back connected on the AT&T network. 

As @formerlyknownas stated, when transferring a sim card into another device you'll need to make sure the device is not locked from AT&T network and is compatible.

Once inserting your SIM card into a non-compatible device, all cellular data connections will be disabled due to the network detecting a device ineligible for use. At this time, you'll need to replace your SIM card for your device as the old one is no longer good.

Also, we're phasing out our 3G network in February 2022 to make room for an even better one that will improve your experience. Feel free to review and check if your device is still available for use on our network. 

We hope this information helps and let us know if you have any other questions or concerns. Thanks for reaching out to the AT&T Community!

Lynn, AT&T Community Specialist

@formerlyknownas The original (the phone the sim is normally in) is an unlocked Oneplus 7, bought from Amazon. So not a proprietary AT&T phone. 

The phone I tested is a Samsung S6 Edge, but I'm not sure if it's unlocked as I bought it from a friend years ago. 

@ATTHelp Is replacing the SIM the only option? Would I not be able to just call AT&T and have them resume the service? 

Hello @krappa,

Thank you for reaching back out to us.

If either of the cell phones that you're using are non-AT&T branded devices, you may need to program your data settings accordingly to connect to AT&T data services.

You can follow the steps at this link to program a non-AT&T Android device with the correct data settings.

Hope this helps!

Sean, AT&T Community Specialist

@ATTHelp That site is perfect, thank you. I will update if it works. 

@ATTHelp

Hello @krappa,

Thanks for giving that a try.

Because that hasn't resolved the issue, we would recommend that you replace your SIM card and contact the previous owner of your device to verify if it has been unlocked or not.

Hope this helps!

Sean, AT&T Community Specialist

@ATTHelp On your previous owner comment, there is none. I am the original owner of the Oneplus device. It was the Samsung that I had bought from my friend (unfortunately I have no way of contacting him anymore). 

Does this information change anything or would I still need to replace the SIM card?