You will need an Ethernet cable to connect your SURFboard DOCSIS 3.0 modem to the SURFboard SBG7580-AC Wireless Gateway.

The SBG7580-AC is a gateway (modem + router combo), not a Wi-Fi extender, so it does not support wireless bridging or mesh connections.

If your goal is to extend your Wi-Fi range without an Ethernet connection, you can use a dedicated Wi-Fi extender or mesh system (like the SURFboard mAX) and enable bridge mode on the SBG7580-AC (if supported) and use it as a secondary router with a wired connection.