How to set up Airport Express with Arris modem?

Hi - my Airport Express was working well until Comcast changed the modem to an Arris - now the Airport express doesn't recognize the internet signal (it works when directly connected to the desktop). Comcast claims I need to determine from Apple how to set it up differently to work with the new modem. Never had this issue before - anyone know how to set it up correctly?

Many thanks

Ron