Gold Standard Instructions For Replacing Current Combo Modem/ Router With New NETGEAR - Nighthawk CAX30 AX2700 Wi-Fi 6 Cable Modem Router

I get the feeling however they may be talking about modems that you get from Cox

Are you referring to this line in the first link:  "If you did not purchase your modem new from a third-party retailer but instead purchased or received it in used condition...?"

Cox is just referring to lost or stolen modems.  There may be either a blacklist via MAC or a conflict with a servicing address.  If it's used and won't register, just call Cox to research why.

You can purchase whatever DOCSIS you want from anywhere...just as long as its make & model is approved and not reported as lost, stolen or conflicted.

Normally when you purchase from a Cox Store, the rep (sometimes) should register your account for a "self-install" and when you get home, connect it and wait...it should register.

However, if you purchase from somewhere else, you won't get registered for a self-install.  In this case, just connect, call Cox and navigate to whatever option to register a new device.  This will lead you to a techie somewhere around the country...and not a sales rep (yuck!).

I have landline service with Cox so the rep freely loaned me a combo DOCSIS-landline modem.

After 6 months, the DOCSIS portion of the combo device failed.  I still had my previous DOCSIS 3.0...which had previously registered fine with Cox...but when I reconnected it the second time, I still had to call to register and activate it.

Bottom line:  If you're not registered (scheduled) for a self-install, you'll need to call.

I'm just babbling here but when I first subscribed to broadband service in the late 90s, an installation CD came with the D1.0 modem.  Ahh...simpler times.

Nowadays, it's a conundrum if you don't have a cell phone.  "Call us on your telephone to activate your telephone service!"  You'd have to find a payphone to activate.