Ok I finally tried it –  swapping in a T-Mobile SIM into an Apple iPad Air that was running a AT&T SIM for cellular data –  and you know what, it works!

It was as easy as ording a Bring your own tablet T-Mobile Nano SIM at –

I got the card in a few days, popped out the AT&T SIM and insterted the T-Mobile SIM and set the SIM up for the iPad.

Here are some quick screen shots after I swapped the nano sim. I used a paper clip to pop the old one you, worked well.

First –  screen shot of my iPad with the ATT Cellular Data

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And after I swapped it with the T-Mobile SIM
NOTE: You must DISABLE your Wi-Fi first so that you can get to the setup screen!

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After that, you just have to walk through the menus and select the No Additional Data… option

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Then setup your Cellular Data Info –  never did it ask me for a SS or Credit Card number –  NICE!

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After a few more screens with agreements and information confirmation, you will get this screen

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It does take a few minutes for it to all process on the T-Mobile side but once it has you will get a pop-up on your screen saying that it has been successfully activated.  After that, you are all set!

Disclaimer: I accept no liability for the content of this post, or for the consequences of any actions taken on the basis of the information provided.  I am just sharing my experiences and results, your experiences and results may differ.