I cannot find the New G20 in the mobile app to set it up?

While setting up the Surfboard Central app, please be sure to select the Surfboard gateway, which is ISP connected with integrated Wi-Fi.

Please refer to the link below, where you can find the steps to set up the Surfboard Central app.

http://arris.force.com/consumers/articles/General_FAQs/SURFboard-Central-Wi-Fi-Cable-Modem-Setup

it will not go past the select device model page?

Please try reinstalling the app and ensure that your phone is connected to the Wi-Fi network from the G20, and that it provides an internet connection.

If the issue persists, try installing the app on a different device for the setup.

did you set it up with the browser first? 
If so, you might try factory resetting it and installing it with the app. 

Or just adjust it from the GUI. (through the browser)

No i cannot get into the browser yet. and neither phone will connect to the wifi on the new router. I got through to the app on a new phone but just wont connect to the router

Please make sure if the Wi-Fi names are broadcasting. 

Could you please let us know who your internet service provider is?

Have you contacted your provider to activate the gateway by providing them with the HFC MAC ID found at the bottom of the unit?