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Ever since my downstream channel was moved from docsys 3.0 to 3.1 my speeds refuse to go over 250mbps down. I have the 300/30 package and would average over 400bps down. Using the arris docsys 3.1 modem
Have you tried a factory reset? Most modems will have a hole/button on the back you can push with a toothpick/paperclip to factory reset the modem. This clears the configuration file, which holds the memory of what downstream channels the modem should use. After the factory reset, you will often have to power cycle your router. If a factory reset doesn't fix the issue, perhaps a moderator here can re-provision your modem.
What model modem do you have BTW?
I have the sb8200 and reset it a few times already. I've done a refresh but not sure of ive done a provisioning. My network at home is fairly beefy. Full cisco meraki setup with a ubiquiti mesh wifi network. The only thing that changed through all of these was the docsys 3.1 light turning on for downstream. I read about a firmware compatibility somewhere. Trying to avoid calling support as they will most likely just want to send someone out for something that can probably be fixed remotely