A:Answer I bought a Netgear Nighthawk Model CM1150V and had to attach it to an ORBI Tri Band Whole Home Mesh Wi-Fi w/ 3 Gbps (RBK 50). It works perfectly.
You should call 1 800 xfinity for help before buying the ORBI, or call Netgear and ask them what you should do, (Netgear support 1 888 6384327)
Good luck