The password can be found on the bottom of the router and in the Network Login Details flyer that may have been supplied with it. 
If you have changed the password and then forgotten it, reset the router to the factory settings. This deletes all the changes you have made in the router’s Settings menu (Graphical User Interface). After the reset, you can use the password that can be found on the bottom of the router and in the Network Login Details flyer that may have been supplied with it. 

Follow these steps to reset the router to the factory settings: 

1. Hold down the OPS button on the router for 20 seconds. Some dual-band routers have two buttons; one for each frequency (2.4 GHz and 5 GHz). In this case, hold down both buttons for 20 seconds. 

2. Wait 2 minutes

3. The reset is complete.

 
OPS button on a router

 
Model with two buttons; both buttons must be pressed.