Read the forum guidelines
I used to have 250-350Mbps down 2 years ago but Cox has throttled down some older DOCSIS 3.0 modems to only have access to the essential 150mbps speed. If Cox didn't have a monopoly on internet service providers in my area I'd switch ASAP. Probably just trying to sell more of their junk modem rentals to unwitting customers and take advantage of their lack of knowledge.
I also have an older 3.0 modem that I was using on Cox's 250 plan (for a long time). A few months ago I came across their new certified cable modems document to find out that my modem was now only supported up to 150. Unfortunately Cox seemed OK letting me continue to pay for 250 - I've downgraded to 150 for the time being, but only because I noticed the issue, not because Cox came to me and told me of the change.
Yes, my modem is old and I'll eventually get an upgrade to 3.1, but I'm a little disappointed that Cox wasn't a little more proactive once they changed their standards.
Nope, I've talked with multiple people in tech support, a supervisor in tech support and to the customer retention/loyalty department. My modem has been reset multiple times remotely and I've rebooted it myself several times. They've assured me that my modem is being throttled down and they are not allowing me access to the service which I used to be able to enjoy.....
What's the make/model of your D3?
The CG3000d is rated for the Essential tier of service and slower. The PREFERRED level of Internet requires a better modem. YOURS is 8 downstream x 4 upstream, to give you an idea, MY modem is 5 years old, (I think), It's a DG2460, 24 downstream, by 8 upstream channels. You can find more information on https://www.cox.com/residential/support/cox-certified-cable-modems.html .
Your modem will have to be replaced if you want faster speeds.