new G20 set up

If your goal is to change the Wi-Fi password, there are two ways to do it. One way is through the Surfboard Central app and other is through the web based interface (GUI)

First try to access the web based interface (GUI) to check the page is misconfigured 

To access the interface, ensure your device (phone or computer) is connected to the default network. Then, open a browser and type http://192.168.0.1 in the address bar to access the web interface.

For G20, during the initial setup, you’ll be prompted to create a password for accessing the web interface.

If you’ve been using the web interface to configure your Surfboard gateway and happen to forget the password, I would recommend performing a factory reset on the device. This will restore the gateway to its default settings, allowing you to set a new password for the web interface.

For your reference, please find the link below with steps to access the GUI.

http://arris.force.com/consumers/articles/knowledge/G18-G20-Web-Manager-First-Time-Login 

If you look closely at the link, you'll find three steps involved. In step 2, you can locate your Wi-Fi credentials and edit the information.

After changing the credentials, try to set up the Surfboard Central app using the option "Device is already set up with web interface." It will prompt you to enter the admin credentials, which is the password you created for the GUI.