Orbi mesh - replacing old netgear combo cable/wifi unit

I impulse purchased NETGEAR - Orbi 850 Series AX6000 Tri-Band Mesh Wi-Fi 6 System (3-pack) - White because I was really angry at my x-Year-Old NetGear Cable modem and extender (and I need improved coverage on the second floor).

So, I get home and lo-behold, I find/remember that I have a Xfinity coax cable screwed into the back of the old NetGear combo cable modem/wifi unit. The coax cable is one of those split cables, one goes into the cable box for the TV, one goes into a cable modem.

So, friends, am I stuck? there's an ethernet port on the back of the Xfinity cable box. If I connect that port to the Orbi with an ethernet cable, will that work? 

Do I need to buy a cable modem (no wifi) and install it "in between" the coax cable and the Orbi?

If I buy a cable modem, won't that limit the mbs to the orbi and degrade the performance of the Orbi?

what feedback and recommendations might you have, please?

(this is the cable connection set up we've had since, like 1997)

LarryC