1. Turn your MOFI4500 off by unplugging the power cable. Wait 2 minutes and then plug back in the power cable. Give the device 5 minutes to fully boot up and make a connection to the tower.
2. Check to ensure you've installed the new sim card correctly and that it is the correct size (standard/ full size sim). See how to video here.
3. Check to see if your device is recognizing the new sim card by logging into the Mofi4500 dashboard (user interface) and going to "+Network" -> "Mofi Internal Modem". The top part of the internal modem page will tell you the device internal module (SIM4 or SIM7), IMEI, SIM ID (SIM card #), Phone Number (MDN or Dial Number), and the Carrier ID (the specified carrier network identifier of either AT&T or T-mobile). Be sure the SIM ID (ICCID/ SIM#) matches the number printed on the installed sim card. Also be sure that the "Access Point (APN)" field has the correct APN entered. (It should be either broadband, fast.t-mobile.com or b2b.static) See APN article for this info. The easiest way to resync your device's module with the new sim card is by clicking the "Reset Module" button found on your "Mofi Internal Page". See below example pictures of this below. SIM4 and SIM7 model Mofi4500 V2 are shown. 
4. Factory Reset your device (if troubleshooting steps did not work).
5. If still no connection, Repeat the above steps and ensure you have adequate 4G LTE signal. We only ship sim cards out that have been pre-activated and tested working in office prior to shipment. If your not getting a connection on your device after all above steps, there is a device issue or localized tower/ signal issue, in which we only have so much control over. Best practice at this point (if you have good signal and all configs are correct, would be to re-flash the firmware or run the mofi recovery steps.