Firstly, the sim card in which we sent you was installed in our office MOFI and tested to confirm normal speeds and network connectivity. With that being said, there is no reason why it will not work in your MOFI unless there are incorrect settings or outdated firmware on your device. 

While the installation of the new sim card is rather simple, there are some more steps which may need to be taken to ensure your device connects to the AT&T network. 

We have found that if you have the APN settings on your MOFI to Auto-detect, the device will likely not register correct APN. Once you have the new sim card installed, be sure to log into the mofi user interface and ensure the Access Point (APN) setting located on the "Mofi Internal modem" Page reads: broadband. For whatever reason we are seeing the mofi input a t-mobile apn (fast.t-mobile) which will not allow a network connection to AT&T. Change the APN to broadband and then hit save and apply at the bottom of the page. Once the settings have been applied, turn your device off from power for 2 minutes and then reboot. Once it comes back online, you should be good to go! 

Additional Resources: 
Go to Tech Support and click on either the V2 or V3 Mofi4500 folder depending on the Version Mofi that you have.