Unable to register g34 on SBC app

In this you can try toggling wifi on the device that your using for the SBC app. 

If you see option to continue onboarding on the application you can click on it to proceed further.

You can try force closing app and continue by re opening the SBC app.

On SBC app version 3.8.6 and earlier, the app will fail to onboard when the Gateway is set up with a custom admin password.  Update to SBC app version 3.8.7 resolves this issue.  Must tap the Click here if you have previously installed with Web based UI portal link at the bottom of the setup screen.  

Please try the above steps to complete the surfbaord central app setup and once you complete the application setup you can access modem GUI through the SBC app.

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