Q: How do I cancel an iPad data plan if I lost my SIM card?

• So I've been paying for a data plan that I don't need for over a year now.  Oops.
• I switched from AT&T to T-Mobile a while back and simply popped in my new T-Mobile SIM before de-activating the data plan from the device. 
• I've since lost the original AT&T SIM card.  Oops.

I've been around and around with AT&T on this for months now.  Despite getting AT&T branded payment confirmation emails monthly complete with payment confirmation code, they cannot seem to locate any record of this in their systems and keep directing me to Apple support.

Anyone know what my next move should be?  Thanks!