There is a factory reset button located in the back panel of the device.  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 device, run through the following steps,

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

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

Please follow this link for instructions.