New home spectrum Internet setup issue (modem and router not working together?)

So new home using same internet provider (spectrum). Prior owner left his Orbi router and 1 range extender downstairs. I brought my Arris surfboard SBG6900AC modem.

I reset the router to factory settings. Now when I try connecting to WiFi I see 2 networks: the actual one with the internet connection coming from the modem, and separately the default Orbi network named orbi59. Connecting to the modem one with what seems to be the actual internet has a super weak connection downstairs and far away from the modem, even next to the range extender downstairs. It seems like the modem and router are doing 2 separate things rather than working together.

Is there something I’m doing wrong? Seems like the Orbi range extender isn’t extending the internet throughout my house because the Orbi and modem aren’t working together it seems.