Xifinity router and modem in 1 can i add netgear router to it

Your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. This would be a double NAT condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge or modem only mode. Then use the NG router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the NG router gets from the modem. https://kb.netgear.com/25891/DMZ-on-NETGEAR-routers
https://kb.netgear.com/24086/How-do-I-set-up-a-default-DMZ-server-on-my-Nighthawk-router
3. Or disable all wifi radios on the modem and connect the NG router to the modem, LAN to LAN configure AP mode on the NG router.
https://kb.netgear.com/20927/How-do-I-change-my-NETGEAR-router-to-AP-mode

Try option #3 first.

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Amandagelia wrote:
Netgear r6080 I want to add this to my xifinity in the house to add internet to my garage

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What does "internet to my garage" mean? Wifi? Wired?

The best option with that device would probably be to put the R6080 into access point mode, wired to the router. That model does not appear support wifeless repeater mode.

I say that because the R6080 is pretty well Netgear's lowest spec router. It is unlikely to come anywhere near whatever your modem/router does.

According to Netgear's manual for this device, not always the most reliable source of information, the LAN and WAN ports support only 10BASE-T or 100BASE-TX. That makes it slower than many newer internet services and most modern network hardware.

It also slows down whatever is going on in your local network. Newer devices support at least 1000BASE-TX.

If you can't run a wire to the R6080, you could see if Powerline Ethernet could bridge the gap. It turns the mains circuit into an ethernet link. It might even get an Internet link into the garage. And the LAN ports on a new powerline plug could be faster than the R6080. New plugs run at 1000 Mbps or more.