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.

Thanks for the advice.  One other wrinkle was that I was also working in a D-Link  AC1200.  The D-LINK and SURFboard DOCSIS 3.0 worked fine together with a cable and no setting changes.

Trying to link via cable with ISP coming into the SURFboard DOCSIS 3.0 (#1), cable running to the SURFboard SBG7580-AC (#2), and cable from #2 running to the D-Link (#3) was not stable.  I could not get into routers 2 and 3 to enable a bridge function, disable the DHCP server or assign a new IP address.  I will probably get rid of my older SBG7580-AC and D-Links and look into wifi extender or mesh system.  Know anyone that wants an SBG7580-AC or D-Link AC1200?

Avoid extenders if you’re using more than 1. Just do it right the first time and pickup a mesh system. You’ll have better performance and stability.