Hello,
If you don't have physical access to the router, nor the device is online via RMS. You can factory reset it, if the device has a SIM card that supports SMS, you can restore its configuration to factory settings with a text message. Simply enter your device's admin password followed by the text restore and separated by a space. For example: Password123 restore. Normally this is enabled by default. In case you have physical access to the router, you can factory reset it by pressing the reset button for 5 seconds. Then you will be able to access with the factory credentials. User: admin Password: admin01.
