This is an older model. I don’t believe it is supported directly by Arris. I assume you got it from your ISP, in which case they should have provided the necessary information.

I found the following using Google. It looks like the default is admin/password and the router information should be on the label on the device.

https://www.telserco.com/wp-content/uploads/arris-dg860-users-guide.pdf