Modem suggestions needed

> My daughter gave me a brand new Nighthawk X6 router [...]

   What's its actual model number?  Look for "Model" on the product
label.

> [...] I know that I need dual band and I know that I need both Wi-Fi
> and ethernet connectivity [...]

   Those are all router properties; nothing directly to do with the
modem.

> [...] Cox internet preferred 150mbps service. [...]

   Practically any current cable modem (model CMxxxx) should be adequate
for 150Mb/s service.  I wouldn't restrict my search to Netgear products.