Only Allowed Panoramic Modem & Its Extender Pods . . . .

Okay, the modem I have is Voice/Internet Modem Brand: CISCO Model: DPQ3212.  I got it back when the telephone first had to have a modem.  The tech said it was an outdated modem, and I noticed some place on the account equipment page there was a notice that it was outdated.  I also noticed somewhere on the website that it stated Cox did not provide service to a voice modem that was not issued by Cox. (I assumed they meant service service, but maybe they meant repair service?) I thus assumed that meant I must get the one and only voice/internet router Cox showed I should get--the Panoramic Gateway Modem/Router.  

Sorry, I don't understand at all what you are saying about eMTA.

My current router is the Netgear Nighthawk X6S (R8000P)--AC4000, and the extender is the Netgear Nighthawk EX8000.  After posting, I came across a web page showing what extenders worked well with the Cox Panoramic modem and the Netgear Nighthawk 7000 was listed, so it seems the 8000 should work, too.

I called for service because the phone and internet keep going out, and I had to either unplug everything or reset the modem, using the reset button on the back.  The funny thing is the tech they sent out said he lived nearby, had hooked up all his Cox stuff himself, and he was currently having the same problem, as are many others, he said.  He thinks it's the weather. 🙂

when the telephone first had to have a modem

This is when Cox changed their telephone service to VoIP.  Cox Digital Telephone (old service) was circuit-switching; VoIP is packet-switching.

The tech said it was an outdated modem

An eMTA is the telephone modem.  The DOCSIS portion of your DPQ3212 is DOCSIS 3.0.  The newest version of DOCSIS is version 3.1.  Yes, the DOCSIS-side of your DPQ3212 is "outdated"; however, you only have a 50 Mbps plan.  I still use a DOCSIS 3.0 because it has been more reliable and online since I got rid of the Cox-issued DOCSIS 3.1.  I haven't had a problem with connectivity since I installed it.

Cox did not provide service to a voice modem that was not issued by Cox

Again, you can't buy a telephone modem at Walmart and connect it.  Cox must issue the telephone modem.  If you're getting telephone service, there is nothing wrong with the telephone-side of your DPQ3212.  As Mouth recommended, just go freely exchange it.  Don't get duped into renting their 3-way combo device (WiFi Panoramic).  You don't need it.

WiFi extenders connect to your router...not the modem.  Look for extenders compatible with your Nighthawk.  The only thing you need from Cox is the telephone modem.  That's it.  Stop shopping for Cox-compatible stuff.

I don't think there is anything wrong with your equipment.  It's just the service to your house.  However, you can rule-out your DOCSIS by freely exchanging it.

Wrong you can buy cox will not activate it