Gigablast requires Panoramic Wifi Modem? That's what she said.

I have just been through the most frustrating ordeal. I swear that Cox uses the "runaround", and dropped calls on hold just as a way to make people give up. Here's the short version (the long version consists of long hold times with disconnects -- nothing says "my customer is important" like having them hold for 40 minutes and then disconnecting them, long chats, texts, and more calls):

When trying to switch from Cox Panorama modem to Arris SB8200 DOCSIS 3.1 32x8 (purchased from BestBuy, not Cox), Cox Tech Support told me that my account was somehow "special", and that I HAVE to use a modem rented or purchased from Cox. When I told them this was NOT the case, they gave me a number for "customer retention". 

Retention told me that the ONLY SUPPORTED MODEM for Gigablast is the Cox Panorama, which of course, is only available for rental from Cox at 10.99 a month. This is BLATANTLY FALSE! I confirmed the SB8200 was Gigablast certified before purchasing it. Here's the link of certified modems. https://www.cox.com/residential/support/cox-certified-cable-modems.html. The "retention" agent told me that must be an out of date document, that the "system" says Panorama is required.

So, I can't use my own modem (which would pay for itself in 8 months of rentals), instead I have to use the Panorama with Wifi disabled (I bought Eero mesh and Arris SB8200 to replace the Panoramic). Is anyone able to help me with this?