Could you let us know the product’s model number to assist you further?

I have same problem. Device is SBG7400AC2 and is brand new.

Since you unable to access the 5g network. Is it only with particular device or all the devices?

If it with all the devices, I would suggest to change the wifi credentials and try connecting the devices again.

http://arris.force.com/consumers/articles/General_FAQs/SBG7400AC2-Changing-the-Default-Wi-Fi-Name-Password

Thanks, but that doesn’t work. I enter admin and password and am told that this is an invalid user name or password - I never get to the screen where I can change the wifi credentials.

If the password "password" is not working, please confirm if the login credentials were changed previously.

If the password is shown as invalid, I recommend performing a factory reset to restore the default password, allowing you to customize it again