There is a factory reset button located in the back panel of thedevice.  Please see image below:

You will need a thin and robust unfolded paperclip (or similar)in order to depress the button.   

To factory reset the devide, run through the followingsteps,

Step 1 Power up the device and wait for 1minute. 
Step 2 Press and hold the reset button until theSTATUS LED turns off. 
Step 3 Release the reset button and wait for 1minute. 

Once the DSL-G624T has been factory reset, all settings willdefault back to factory settings.  You will then need toaccess the device using its default IP address of(192.168.1.1) and reconfigure it for InternetAccess. 

Please follow this link for instructions.