You likely will need to contact ATT tech support and ask for the "PUK" unlock codes for your SIM. Along with the time limit change, AT&T locked the SIM so it couldn't be moved to another device. TS is usually pretty good about giving out the codes. They will give you one code that can be used right away and a second one for future use if needed. It's been a couple of years since I needed to use a PUK code, so maybe someone else will jump in and with a more complete answer.
Oh, and I'm 77, so even us old guys can learn new tricks if we put our minds to it... :)