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JulieAM's avatar
New Contributor

Slow Internet After Upgrading Modem


So more than a few months back I upgraded my SB5100 modem to a SB6121 modem and my speeds actually decreased.  I was routinely getting around 60/10Mbps before I upgraded the modem (I'm paying for 50/5Mbps), and now I get about 25/6Mbps.  The tests were performed right at the modem (hardwire connection).

I didn't put up too much fuss in the beginning, because I figured maybe a firmware update or config file would be pushed to the modem to fix this issue, but it's been more than a few months now and my speeds haven't improved.  I've tried using the Cox Modem Reset tool with no change; I chatted with Customer Service on two separate occasions and all they did was reset my modem, which resulted in no change.  I bought a new modem (a Netgear AC1750 R6400) which has great ratings, and that didn't improve my speeds at all.

These internet speed test results, are the results I'm getting when using the Cox speed tool, the Google Speed Test, and the website... I try not to use, as I've heard flash speed tests aren't reliable.

I'm kind of at a loss as to what to do next... I feel like I should have never "upgraded" my modem, as that's when the problems started.  I contemplating just downgrading to the 15/2Mbps plan, as that's pretty close to that speeds I'm getting.

Any advice?  Thanks!

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  • AllenP's avatar
    Valued Contributor

    Getting 60Mbps with a DOCSIS 2 modem (SB5100) is strange, if not impossible since the published max speed is 38Mbps. What you are saying is you were getting almost twice the speed the device is capable of.  The SB6121 is a 4 channel DOCSIS 3 modem capable of 4x the speed of DOCSIS 2. There may be a service code issue on your account indicating you still have the old D2 modem ... can a Cox mod check that out.

    The Netgear AC1750 R6400 is a router, not a modem. If you are getting 25Mbps wired to the modem (SB6121), upgrading the router won't help. Hopefully a mod will chime in and check your account setup. In the meantime, you may want to check the signal levels at and post a screenshot here.

    Edit: just checking back, about a year ago (Oct '15) you posted the same thing ... upgraded to DOCSIS 3. The result was you were getting correct speeds direct to the modem but your router was the bottleneck. Is this a continuation or a new problem? Back then, DustinP from Cox offered to check your account setup, did you ever email them you info as requested?

  • Hi JulieAM,

    The service codes on your account look fine, as do your modem levels. Are you able to provide examples of your speed test results? Make sure that you run the speed tests using a device that connects directly to your SB6121. (Leave the Netgear router out of the picture for testing purposes.)

    So that I can take a look at how the modem is functioning on the Cox network, please follow the steps at and log into to run the Cox Internet Speed Test Tool. I want you to get the speed of the internet tier to which you subscribe!