An old SIM card was replaced with a new one and it stops working

Check that the new SIM works in a phone. Check you can make a phone call with the new SIM. If the device appears to be operating normally but inserting a SIM from a new provider has caused it to stop, then using the old SIM, send the INFO command to obtain the firmware version number and note the response. The firmware may require updating to support the new SIM. Contact QTech for assistance.